US2181133A - Lecture desk - Google Patents

Description

Nov. 28, 1939.

D. KATZ LECTURE DESK Filed Feb. 2e, 193s :s sheets-sheet 1 INVENT'R Nov. 28, 1939. D, KA-rz I 2,181,133

' LECTURE DESK Filed Feb. 26, 1938 3 Sheets-Shet 2 INVENTOR Nov. 28, 1939. D. KATZ 2,181,133

LECTURE DESK Filed Feb. 26, 1938 3 Sheets-Sheet 3 A5 X f Patented No'v. 28, 1.939

PATENT oFFlcE LECTURE DESK David Katz, Wilmington,v Del.

Application February 26, 1938, Serial No. 192,755

12 Claims.

This application is a continuation-in-part of `my copending application Serial No. 155,390,

led July 24, 1937, and relates to certain novel lecture desks which are provided with means for displaying to the audience writing or sketches done by the lecturer during the course of his lecture. f

More particularly, this invention deals with a novel lecture desk whichenables the lecturer to illustrate his discussion with writing or sketches,

and this shadow is focussed, with suitable inver.

`sion and change of direction, by an optical system within the interior of the desk chamber, onto a projection screen placed in a substantially vertical position in a cut-away space in the rear y wall of the desk, that is the wall normally facing the audience. The Iprojection screen is made of -3'0 shadow transmitting material, whereby the shadowsfocussed upon it from the inside of the` desk show through to the outer face thereof, confrontthe descriptionpmceeds.

ing the audience.

In the model described in my copending application, the writing panel consists of a at sheet of translucent material.` 'I'he latter may be either ground glass or a sheet of translucent paper; but in any event, the faces of the writing panel are two planes parallel to each other. 'I'he 40' optical system, which consists of a convex lens and a plane mirror, or a systemcf lenses and mirrors, depends for its operation on the scattered light which reaches the lens from the various particles in the translucent writing medium.

Now, I nd that the above system gives satisfactory results only where the source of illuminationhas a wide area compared to the aperture of-the lens employed, as for instance where a number of electric lamps have their light reflected uniformly unto the writing plane by the aid A of a reector of large effective area. Such a system is practical with small models. With larger models, however, the required size of the reector area .tends to diminish considerably the practical adaptation of theldevice, On the by my present invention which consists, in genother hand, if the requisite illumination is permitted to come from a relatively small area, the projection on the screen is found to contain a bright spot in the middle of the screen area, with practically no illumination, and hence no shadow 6 elects, in the'surrounding regions.

It is accordingly an object of this invention to provide an improved lecture desk of the general type above indicated which will give satisfactory results even with a lrelatively concentrated source 10- of light. A further object of this invention is ,to provide an improved article of the above general type wherein the usable portion of the projection screen will be substantially uniformly illuminated lthroughout its expanse, with resultant good 15 lightand-,shadow contrast, regardless of the' shape or size of the illuminating means employed. .A still further object of this invention is to improve the design of my lecture desk aforesaid whereby it may be made to give satisfac- 20 tory results with less strong illumination than is vrequired with my prior model, resulting not only in-a saving of electric power but also in greater comfort to the lecturer in that hel need not be exposed to the radiant heat from a large 25y cluster of lamps. Y

A- still further object is to provide a design which lends itself more readily to modiiication for the production of large size models, than was my former design. Other and further im- 30 portant objects of this invention will appear as These and other useful objects are achieved eral terms, of replacing the heretofore employed 3g doubly plane writing panel by an optical member which is adapted to combine the functions of a writing plane with those of a light-condensl ing lens. In other words, the cut-away opening in the top of the desk, instead of being lled by a panel of glass which is plane on both sides, is according to my. present invention, lled by a member of glassy material which is flat on they upper surface thereof. and serves there as a writ` ing plane, but is either shaped like a convex lens on the lower side or'else combined with a condensing lens of any suitable form, wherebyv to bend light rays passing through it, into the general direction of the optical system below. t

For a better understanding of my present invention, reference isto be made to the accoml panying drawings which; form an integralpart desk built according to this invention, and employing a plano-convex lens as writing panel.

Fig. 2 represents a similar section through the foremost top portion'oi the desk in a modiied form of this invention, employing an annular Fresnel lens.

Fig. 3 is a 'vertical section of a still further modied embodiment of my invention.

Fig. i is a perspective view, partly in section, of the lens member employed -as writing panel in the desk of Fig. 1, on a slightly enlarged scale.

Fig. 5 is an enlarged sectional view of the annular Fresnel lens employed as writing panel in- Fig. 2.

Fig. 6 is an optical diagram, showing 'in simplified fashion one mode of disposition of the illuminating'` means, writing-panel lens, focussing lens ard projection screen relative to each other.

Fig. 'l is a similar diagram showing a different mode of relative disposition.

Fig. 8 is a similar diagram showing a still different mode of disposition.

Fig. 9 is a similarv diagram showing a modied form of disposing the illuminating means in any of the aforegoing'designs.

Fig. 10 is a simpliied optical diagram showing the focussing of the points of shadow which come from the writing plane unto the projection screen.

'I'othe weight (or spring) 62 over the roll 6I at the ing the interior. The inner partition has for the same reason been partly broken away.

Fig.. 12 is a diagrammatic pictorialization of a K modied form of the optical system to be employed in commotion with the embodiment displayed in Fig. 11.

Fig. 13, is a similar, three-dimensional diagramof a still further modied form of the optical system. e

Fig. 14 .is an enlarged perspective view of the tilted-mirror system employed in Fig. 11.

Fig. 15 is a section either through line X-X or through line Y-Y in the writing panel of F18. 11. l

Referring now to Fig. 1 in detaili, l constitute the walls,.t op and floor of the lecture desk which are made of opaque desk material, and are joined together to form an enclosed darkchamber, as in my former application. 2 is the writing panel, Awhich will be described in detail later. 3 is the projection screen, made of shadow transmitting material and supported pivotally at 3|, with/means for adjustment at 32. I is the focussing lens and 5 is the mirror of the optical system, both being-supported lon a suitable support 'l which is adjustable in position through screw 1I and slit 'l2 in the wall of the desk; nner adjustments of the lens being obtainable by the aid of the threaded tube 42 for the lens, and axis 5| and set screw 52 for the mirror. I

Under the eave of the desk on the side neares the lecturer (or at any other convenient location) is a `ro1l of light-transmitting paper 6I, from which a sheet 6 stretches over the surfaceof the writing plane 2, and is held in tension by the pivoted knife edge device Biron one end and by other ends. The desk is further provided on the from being scattered unto the projection screenl at the bottom thereof to permit the principal' beam of light to pass into 'the lens 4.

So far the novel lecture desk is substantially identical with the desk described in Fig. 2 of my copending application, Serial No. 155,390. The

are not inventive differences, being within the scope of my original invention above referred to, But the principal difference between the desk of Fig. 1 of this application and the one of my copending application is in the nature of the writing panel 2.

Instead of being a flat pane of glass as in my previous design, the writing panel in Fig. 1, here, is a plano-convex lens. particularly the upper surface, is at, as shown at 26 in Fig'. 4, and forms the writing plane. The other surface, however, is convex, as at 2l, and is of suicient curvature to bend the light rays coming from the illuminating means i5 into the general direction of the lens'. Lenses of this type are knownsgenerally as condensing lenses.

Precise focussing is not needed, and the location of the focus may be either in advane of or beyond the lens The aperture of this lens, that is its width One surface of it, more from edge to' edge, may be anywhere from about 6 to 24 inches, depending on the size of writing plane desired. Its outlinemay be circular, or lit may be vsquared o i, as at 2l, 2| in Fig. 4. The material of the plano-convex writing panel may be glass or any suitable, hard, transparent material, for instance Lucite (a methyl-methacrylate resin) Or it may be compounded from two diiferent materials, for instance glass and Lucite, crown glass and iiint glass, as is customary in lens design. But the thickness of the glass should be kept .down to a minimum, inasmuch as a thick lens is apt to distort the shadows on the projection screen. For this reason, the radius of curvature, and consequently the focal length, of the condensing lens should be as large as possible. A

The writing-panel lens may be lightly ground on its at surface, to be written on directly with a soft pencil or crayon. But it isl preferable to have this lens clear throughout, and to use a sheet of transparent paper as the actual writing surface. On th other hand, extreme optical perfection,such as freedom from aberration, is

not essential to its performance in the present invention.

The lens is mounted in the top of the desk so that its plane face 26 issflush with the top lsurface of the desk, to provide a continuous flat support for the writing paper. Since the width of thelens is for economical reasons kept down to a minimum, the writing is preferably done with a lead pencil or sharpened crayon or ink, so that, the gures and letters are relatively small. 'I'he optical system within the desk, and projection screen can be designed to effect any desirable degree of, enlargement.

Cooperating with the plano-convex lensis a source of illuminationi l5, xed at a definite distance above the writing` plane by means of an adjustable support I I. 'I'his support may be independent of the desk or may be attached thereto, but to save .the necessity of delicate adjustments in installation, the lamp support is preferably designed and built as a part of the desk. 'Ihe lamp l5 is provided with a suitablereilector the partition l is provided with an opening Il |0, preferably bearing-.at its end a condensing u is shown diagrammatically in Figs. 6 to 9, in-

clusive, wherein the plane mirror 5 has been omitted for simplicity, inasmuch as it does not ailect the principles of this discussion. Omissionl of the mirror enables us to represent the light rays travelling fromthe writing plane to the screen by straight lines, although in actual practice they are broken lines. fore that in actual practice the distance between the focussing lens 4 and the projection screen 3 is to be measured along the broken paths of the light rays instead of along the shortest path between the two. Flgs.6 to 9, together with Fig. 10, also bring out the quantitative relations in volvedl and aid one in selecting the proper dimensions and design data for any particular A purpose.

In Figs. 6 to 10 inclusive, W represents the writing plane, or condensing lens used as writing panel, as well as the diameter thereof. L represents the shadow focussinglens, kas well as the diameter thereof. S represents thel projection screen as Well as the diameterthereof, assuming that the screen will be Acircular and just of the right diameter to accommodate the image 'pro- .iected from the field of W. In case W.and S are rectangular or square, -these letters may be considered as representing the length of their respective diagonals. O represents the source o'f illumination, for instance the lamp I5 in Fig. lfor an arc light.4 It will be further assumed in these gures that f is the focal length of Lwhile F is l beyond L as in Fig. 8. Regardless where this the focal length of W.

Now, considering rst Fig. 10, it will be observed that each opaque point in the writing plane W emanates a beam of shadow which is condensed by the focussing lens L and focussed upon the screen S as though it were a ray of light.

This focussing effect, and the position and size of the image, are utterly independent of the position or strength of the light sourceO, and are also independent of the focal length of the lens W. Consequently, the relations shown in Fig. l hold true with any of the modifications, as to mode of condensing the illumination, shown' in Figs. 6 to 9,

inclusive.

If S is of the correct dimensions to just accommodate the image of theentire field ofvW, then S P y From these formulas the requisite dimensions of -ables are assumed or determined by the practical requirements.

Referring now to Figs. 6 to 8, the focal length F of the writing panel W may be so chosen, and the light O may be located at such a distance, that the rays of light therefrom, after being'bent by the lens W, come to a focus at a point in advance of L, as in Fig. 6; in the center of L, as in Fig. 7.; or

point of focus is, and regardless indeed whether there is a sharp focus at all or merely a rough condensation of the light within a small region,

It is to be remembered therep and q in turn are related to the focal length of .lens L by the well known lens formula all the light that has passed throughv W will be just spread out over the area S (as definedby Formula 1), without loss, provided L is of sucient width to intercept all these rays. 'I'his condition will be realized if in Fig. 8.

Itis clear that any value forl L which lies between those obtainable by Formulas (3) and (4)v will be satisfactory. In all these cases S will be intensely and uniformly illuminated, since all the light from W is spread out upon S, without loss.

However, if the distances q and a: be assumed as fixed, the layout of Fig. 6 will require W to have a shorter focus than in Figs. 7 and '8. The

ldesign of, Fig. 8 will give the maximum focal in 6, and

inFig.8. l l

The relation between y and a, in eithercase, is

- determined by the focal length F of the condensing lens W, by the simple lens formula:

This one when combined with and (6) gives:

in Fig. 8. VThese formulas help select the proper dimensions for the various variables involved in the design of the desk, as more fully illustrated by the following discussion.

In practice, q will be limited by the height of thelecture desk to a value between about 2 to 3 feet, at the maximum. F cannot be greaterthan y, as determined by Formula (6). If Fis to be equal to y, a: in Formula (7) must be infinite. In other words, the writing plane should be illuminated by parallel rays of light. Formula (9) then becomes converted into 1 f 1 L f gx-W) (1) On the other hand, it is not practical to place the light too close to the writing plane, and, as

"already mentioned, it is not desirable to have a small F value forthe lens W. Both of4 these factors suggest setting a lower limit to the value of for instance,-a value equal to that of y. In that event, Fis just abouti@ y. The most practical values of a: will fall somewhere between these two extremes, which will locate the value of F somewhere between 1/2 yand y.

To make F as near as'possible to its upper limit,

' lens C increases the value of a without bodily removing the lamp to an impractical distance. This modification also gives a sharper definition of image and a greater contrast between writing and background, rendering the projection more easily readable from a distance, even in daylight. By placing Ov at the focus of C, the emergent light rays may be made actually parallel; (.r= However, strictly parallel rays would requireC to be of the same aperture as W, which is again objectionable from the viewpointof cost. Therei lfore, in practice, some divergence is permitted,

and a: may be taken as having a value somewhere between y and 3y. A specic illustration will make the discussion clearer..

Assume that q is to be 30 inches, and that the aperture of L is to be about V3 that of W. Then, by Formula (10), the maximum operative value of F is 45 inches. This is also the value of y in Formula (6). If :c be taken' as2y, =90 inches. Then, by Formula (9), the practical value of F is 30 inches.

The radius of curvature of a plano-convex-lens is related to its focus by the formula 'Reel-151e` (11) wherein n is the index of refraction. Assuming a material for W whose index of refraction is 1.7

(flint glass), Formula (11) becomes R=0.7 F, whence, in our problem, R=211 inches.

Assuming now that it is not desirable to have' the lens W'more than Zinches-thick at its maxi' mum, the'permissible aperture of `W is calculated readily by plane geometry. 'I'he formula is where tV is the maximum thickness of the,lens, and R is the radius of curvature. Applying this formula, W=about 18 inches..

Now, suppose that a magnification of 2% is de-l sirable in the projection;` then thev required diameter of.S is 21/2X18=45 inches. Of course, the actual screen may be larger than the found value, and may be rectangular, square, or any .other shape, provided itv accommodates the de- L. In practice, a stock size lens, approximating the determined focal length, will be selected for L; and the value ofp will then be adjusted to suit. In the instant case, Formula (2) suggests a lens of about 21 inches Ifocal length, which in turn reduces the value o f p, by the same formula, to

70 inches. Its aperture, from the previous assumption.

is. to be about 6 inches. Here again. a larger l width does no harm. Hence a 7 or 8 inch lens may be selected if desired.

In the same problem, if athreefold magnication is desired, a lens of 22.5 focus is required for L, and the screen should be not less than 54" on a side.

'I'he value of was assumed to be 90". A practical height for the illuminating means would be about ft. above`.`the writing plane, which would make it suiciently high to clear by a comfortable margin the head of a tall person standing Bm a platform in front of the desk. Then, by plane geometry, the aperture of thelens C must be about 6 inches. I

Of course, in lieu of a single element of light with a flash-light type lens in front of it, a fixture of f considerable dimensions, employing several lamps may beused. In that case the impinging light rays may be made substantially parallel, with the result that F may be used to its full 45 incheswln that event, R is about 31.5"; permissible W, about 22,'when t is 2, and about- 24.5" when t=2.5,

In the same problem, if crown glass or Lucite" tained only by increasing the thickness to 3 inches, which still is fairly reasonable. On the other hand if a 2-inch thickness is insisted on, W must be reduced to about inches. A maximum thickness of 1% inches with crown glas-s will give a permissible aperture of about 13 inches.

In practice, it may sometimes be desirable to select a stock`lens for W, and determine q and the other dimensions accordingly. For instance, a crown-glass, plano-convexcondensing lens of 14" aperture and 21" focus is carried in stock by some manufacturers at this date. If it is desired to build a lecture desk according to this invention from this lens, y is rst determined from Formula (7) Assuming that the lamp is placed at a distance of 5 feet and uses a condensing lens of-5" aperture, :c is about 93 inches. 'I'hen y is about 27". Assuming now an aperture for L of 5", Equation (6) gives q=about 17". A threefold magnification would then require 21:51"r

and S=42". The requisite focal length 'of L would then be about 13".

In the same problem, if parallel light is' employed' (:c=), `then y=F=21",fI q==13.5",

p=about 40", S=about 32", and requisite f=10".

Ifin the same problem, the aperture of vvL is reduced to 4 inches, q becomes about 19", p about 5'7", and the requisite f=14". It is clear that all of the above designs lend themselves to practical use.

It was stated above that q is generally limited by-the height of the table; this is due to the fact that the writing plane is substantially horizontal and that the lens l is substantially vertically beneath it. However, such an arrangement is not an absolute necessity. If desired, the mirror 5 may be placed in advance of the'lens 4 instead of following it. Then the light rays will become vbroken .into' horizontal direction before they` 'strike the lens, with the result that the lattercan be placed at any point along the horizontal I path of the raysl. Consequently, the distance q can be increased to any desirable dimensions,

with' resultant beneficial eiectupon the permissible value of F and consequent permissible width of W.. This mode lof disposition is therethen interposed between the focussing lens and the projection screen.

`A-diiferent mode of increasing the size o'f the writing plane is illustrated in Fig. 2.

In this modification, the lens 2 is notan ordi-v nary condensing lens, but what is known as an annular Fresnel lens. An enlarged section of such a lens is shown in Fig. 5. The ribs on the under surface' of this lens run circularly around its center. The details of such a lens are well known, and need no detailed description. See, for instance, Encyclopedia Britannica, 14th ed., vol. 14, page 91. IIt is to be remembered, however, that' because ofthe thick portions of the writing-panel lens tend to displace'the image somewhat, and because the thicknessof a Fresnel lens changes abruptly at its-ribs, such a lens is apt to project upon the screen a series of concentric rainbow-colored rings, disrupting the continuity of the image. This undesirable eifect can be'weakened by using monochromatic light and by increasing the number of ribs, thereby decreasing their height differences at the points of abrupt change. In any event, the utility vof this particular design is limited, and it is therefore not a preferred embodiment.

rn Fig. 3 I have shown stili aA third moda' iication of my invention, in which a doubleconvex lens is used in conjunction with' an ordi'- nary plane plate of glass to give the combined functions of the plano-convex lensof Fig. l., The double-convex lens is placed underneath the A writing plate, and as close to it as possible, as otherwise it is apt to distort the image somewhat.

'Of course, in lieu of'a double-convex lens, a

plano-convex lens with its at face either up-` wards or downwards, or any other form of con-f densing lens, may be employed.

In -a similar manner any other combination of lensesfprisms. etc., with a plane transparent surface may be employed, provided thecombination produces the following two eilects:

(l) provide a flat support for the writing paper to be Written on, and v(2) bend the light rays coming from the illuminating source, into the general direction of the focussing lens 4.

It will be clear that in lieu of the simple focussing lens 4, a combination of 2 or '3 lenses in series may be employed, provided these are placed close to each other so as to effect altogether but one inversion of the image. Such compound lenses are quite common in the artof projection, and are designed to eliminate spherical and chromatic aberration. For all purposes of this invention such compound lenses are to be regarded'as the equivalent of the lens 4 (or lens L) shown in the various figures and diagrams.

, Turning now to Figs. 1'1 to 15 inclusive, these g'ures show several modii'lcati'ons of my invention whereby it is' possibleto increase the size of y trimmed down to straight lines, as shown inde# tail in Fig. 4, in order that each may ilt 'inv l 5' properly with the other to form a continuous transparent plane. This is explained further .by Fig. 15, which constitutes a section through the writing plane of Fig. 11, both 'along' the line' X-X, and along the line Y--Y.

In such an embodiment, fourfocusslng lenses are required in lieu of the single lens 4.of Fig. l,

each of these focussing lenses 45, 45 cooperating with a corresponding condensing lens from -the group 25, 25. Now, if a plane mirror were employed `with this modification of my invention, the projection on the screen. 3 would -be scrambled, because the writing from quadrant A would'be projected into quadrant D, that from B into quadrant C, etc. To prevent such dis-l astrous results, the plane mirror 5 of Fig. 1 is replaced in Fig; 11 by a system of. four mirrors 55, 55, which is shown more clearly in Fig. 14. The four mirrors are supported on a single plate 5l, which issupported in inclined position in the same manner as mirror 5 in Fig. l so as to direct the shadow rays in the general direction of the screen 3. Buteach mirror on this support vis lifted on its outer corner, for instance by the aidJ of adjusting screws 552 56, and tilted inwardly toward the center of `the support.' TheI tilt of each mirror is adjusted-sumciently to cause the shadow rays from quadrants A, B, C and D of the writing plane to be projected, respectively, into the correctquadrants A, B, C and Dof the screen 3, and to cause these quadrants of projection Vto join up correctly at the edges with'y out overlapping.

In this modification, the light source I5 is likewise replaced by a ixture'of .four individual light sources |55, |55, each cooperating with one of the qua writing plan and each provided with a reflector or condensing lens (not shown) designed to dil regt the ,light toward the center of the corresponding quadrant. A

The paths of the rays of light and their cross reflection by the mirror system can be traced in Fig. l1 by following the central light rays a, b, c and d.

Apart from the features' above described, the modification f Fig. 11 follows in general the design of Fig. l, and mayhave all the details or variations of the latter. For instance, the projection screen may be inclined as in Fig. 1. A Also," the mirrors 55 need not be supported on the. same plate', but may be supported each with its respective lens 45 on anl individual support, provided each mirror is placed at the correct angle to reect the light into the corresponding quadrant of the projection screen. The mirrors may also precede the focussing lenses 45, 45, in which event the latter Iare placed substantially vertically, along the horizontal portion of the path of the rays of light, as already explained in connection with Fig. 1. y

Fig. 12 is a modication of the embodiment shown in Fig. ll, in respect to the mirror system. Instead of four tilted mirrors, a plane mirror 5 is here employed, placed at an angle just as in Fig. 1. But after being reflected from the mirror..

ants A, B, C and D of the composite the light rays are retracted by means of a y system of prisms 9 comprising four prisms 95, 95. Each ofthese uprisms is bound by plane faces and tapers from the center of the system 9l toward one of its corners. llnqthis fashion, the prisms refract the light rays a, b, c, d and direct them toward the quadrants A, B, C, D in correct order. Y A

Fig. i3 is a further modification of the relfleeting system of Fig. 11. Here, the mirror consists of two plane mirrors 56, 56, arranged in trough-like fashion on the common support l,

with the axis of the trough running transversely of the writing desk. Cooperating with this mirror is a prism system 9 consisting of two wedgeshaped prisms 96, 9B placed back to back, with their thin edges in a vertical direction. The action of this compound system on the light rays can be followed readily by tracing the lines a, b, c, d in this figure.

The modification of Figs. 11, 12 and 13 is adapted also -to be used with a writing panel which is a flat sheet of glass, as in my parent applicationinstead of a system. of condensing lenses. This modification, however, requires the lamps |55 to be spaced farther apart, so that each lamp is in line with the center of the corresponding quadrant on the writing plane and the corresponding focussing lens 45. Also, this modification, requires strong illumination coming from a source of wide area. It is therefore not a preferred modification.

It will be understood that the specific embodiments described above are merely illustrative and are not intended to limit the invention.' Many variations in the details of construction may be made without departing from the spirit of this invention.

For instance, the lecture desk may have a floor built in therein, or may be made without a oor. 'I'he illuminating means maybe a part of the desk, or may be omitted, to be installed locally I in the auditorium with the installation of each unit. The partition 8 in any of the modifications may take any of the forms shown in the drawings of this application, or may be omitted altogether. Likewise, the projection screen 3 may be placed inwards of the front wall of the desk, as in Fig. 1, flush with it, as in Fig. 3, or may be moved out to the end of the hood I6 or other similar light-excluding extension built onto the desk. f

Instead of a single lens and a single mirror in the optical system, anyA combination of lenses and mirrors may be employed which, in addition to focussing and projecting, will eiect one net re` versal of the horizontal dimension of the lecturers writing and a net zero reversal of the top-` to-bottom dimension thereof, thereby orienting the projected script on lthe screen in correct manner for reading by the audience. The projection screen may be made of fine cloth, ground glass, translucent paper, or any other material which will transmit shadows through itself while simultaneously scattering somewhat the light passing through itself. Many other variations liquid. .For instance a large watch glass of spheroid curvature maybe cemented around its periphery to a plane sheet of glass to form `a plano-convex shell. A vent provided at a suitable point, communicating with a partially lled, sealed reservoir, may provide an outlet for 4 thermal expansion and contraction of the liquid.

Of course, the liquid for such a purpose should be chosen of high boiling point, say over 80 C.,

.75.' low freezing point, say below 0 C. or below -10 C., and convenient refractive index, say between 1.33 and 1.75. Water seems to satisfy these conditions to a fair degree. Other con venient liquids may be selected from physical and chemical tables.

1533 and 1535.

As for the sheet of paper 6 which is stretched See, for instance, Handbook of Chemistry and Physics, 19th edition, pages 1526- over the writing panel 2, I find that the more transparent it is the better the contrast between writing and background on the projection screen. Best results are obtained with highly transparent artificial cellulosic sheeting, for instance nonmoistureproof y ,Cellophane".

I claim as my invention:

l. An apparatus for projecting handwriting, comprising in a single unit of furniture a substantially horizontal writing panel adapted to support a sheet of light-transmitting writing mal terial on the upper surface thereof, a substantially vertical projection screen, and a focussing system; said writing panel being composed of light-transmitting material and being provided on the underside and adjacent thereto with lightv condensing means, thereby being adapted to con- 'dense and to pass through itself into the space beneath itself light rays coming from a source of illumination above said writing panel through said writing material; said focussing system being disposed between said writing panel and said projection screen and being adapted to project upon the latter the light rays which come from the former, and to focus upon said screen shadows of any writing or figures done on said writing material with a light-absorbing wnfing vmedium; and said screen being composed of shadow transmitting 'material whereby to transmit said projected light and focussed shadows through itself to the other face thereof, thereby displaying the writing or figures as shadows upon an illuminated background; said writing panel, focussing system and projection screen being held together in their proper relative positions` by ture unit being free of any obstructions or rnembers in the line of sight between the lecturer and his audience.

2. An apparatus for projecting handwriting. comprising in al single unit of furniture asubstantially horizontal writing panel supporting a.

sheet of light-transmitting writing material stretched over the upper, surface thereof, a airbstantially vertical projection screen, and afocussing system; said writing panel consisting of a plano-convex lens of transparent material, therei by being adapted to condense and pass through itself into the space beneath itself light rays com; ing through said writing material from a source of illumination above said writing panel; said focussing system comprising'a focussing lens and at least one plane mirror cooperating with each other to project upon theprojection screen the light rays coming through the writing panel while simultaneously focussing uponsaid screen shading material by the aid cf a nent-wending writing medium; and said screen being composed of shadow transmitting material, whereby to transmit said projected light and focussed shadows to the other face thereof, thereby displaying the writing or figures as shadows upon an illuminated background, said writing panel, focussing system and projection screen being held together in their proper relative positions by said single unit of furniture, and the latter being shaped and proportioned to serve simultaneously as a lecture desk, and to formV a substantially inclosed space behind said projection screen,.thereby excluding stray light from the inner face of the projection screen; said plano-convex writing panel being disposed in an opening in the top of said furniture unit with its plane surface uppermost, in convenient positionvfor writing thereon by a lecturer during the course of his lecture while facing the audience; and said projection screen being disposed in said furniture unit with its outer face displayed to the audience while the latter is facing the lecturer.

3. An apparatus as in claim 2 in combination with means of illumination located' outside said furniture unit above said writing panel, said illuminating means comprising a source of light and a condensing lens, whereby to throw upon said Writing panel a concentrated beam of light.

4. A combination as in claim 1, the writing panel being formed'by a plane sheet of glass associated with a converging'lens located immediately beneath the same within the interior of the cabinet, whereby to converge the light passing through said writing panel into the interior of the cabinet.

5. A lecture desk comprising in a combination a cabinet, a writing panel in the top of the cabinet consisting of light-transmitting material, a shadow receiving screen in one of the vertical sides of the cabinet composed of shadow transmitting material, an optical system within the cabinet adapted to intercept shadows coming from the writing panel and to project them upon the screen, said optical system being composed of a plurality of focussing lenses, each of which is adapted to receive light rays and shadows from a separate region in the writing panel, and each being associated with independently adjustable means for breaking the light rays passing through said focussing lens and for directing them upon a 'definite and separate region on the projection screen, corresponding in orientation to the region on the writing panel where such rays originated, the said separate regions and orientations being so selected with respect to each other that the separate projections by the various focussing lenses upon said screen add up on said screen and present to view an orderly and continuous projection of the entire eld of writing laid out on the writing panel.

6. A lecture desk as in claim 5, the writing panel being composed of a plurality of planoconvex lenses tted edge to edge to form by their plane surfaces a continuous writing plane, each ofsaid plano-convex lenses acting to condense light rays passing through itself and to direct it toward the corresponding focussing lens cooperating therewith.

7. An article of lecture-room furniture comprising a desk having a top member and wall members of opaque material forming a substantially enclosed, darkened chamber; said top' member having at least a portion thereof removed and replaced by a writing panelv composed of light transmitting material; and one of the Wall members having likewise a portion thereof `removed and covered up by a shadow transmitting screen positioned to display to an audience facing said desk light-and shadows impinging upon said screen from within said chamber; said writing panel being adapted to cooperate with means of illumination external to the desk whereby to transmit into the interior of the darkened chamber light coming from'said illumination means and to transmit shadows of figures written on the writing plane with light-absorbing writing material; and 'an optical projection system within the interior of said desk placed below the light rays coming from said writing panel, said optical projection system including imageforming means and image-reversing means cooperating with each other to focus upon the inner side of the projection screen an image of shadows coming down from the writing panel and to reverse such image in the horizontal dimension, whereby such image when transmitted through the screen material to the outer surface thereof shall appear to an audience facing the projection screen to have the same up-and-down and right-and-left orientation as the original figures on the writing panel have from the viewpoint of the lecturer.

8. An article of lecture-room furniture as in claimh7, having further means, including a lightconverging lens, associated with said writing panel whereby to concentrate the light-rays coming through the writing panel and to bring them substantially to a focus at a region within the interior of said chamber in the vicinity of said optical projection system.

9. An article of lecture-room furniture as in claim 7, said writing panel consisting of a planoconvex lens having its plane surface substantially flush with the top surface of said desk, thereby to serve asa writing plane. n

10. An article of lecture-room furniture as in claim 7, having further means associated therewith wherebyA to stretch a sheet of light-transmitting paper over said writing panel, thereby to serve as writing paper.

11. An article of lecture-room furniture as in claim 7, said writing panel consisting of a planoconvex lens having its piane surface substantially flush with the top surface of said desk thereby to serve as a writing plane, and said desk having further meansI associated therewith whereby to stretch a sheet of transparent-paper over said plane surface ofsaid writing panel said writing panel in substantial alignment with DAVID 70

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