INCREASING THE USEFULNESS OF THE SHORT FOCUS PROJECTION LANTERN. PART I ROBERT TAW,UNIVERSITY OF KANSAS,LAWRENCE, KANSAS A projection lantern with a short focus lens (in the neighborhood of 4") has several desirable features for presenting data and diagrams to the smaller classes of advanced students ~n chemistry.' The writer has been experimenting for the past several years with various arrangements of lantern and screen and of various screen materials, seeking to obtain the most satisfactory combination for the purpose noted above. As there is but little published information of this nature available, it has been thought worthwhile to publish these notes. Doubtless other teachers could make valuable suggestions from their experiences. The more important requirements of a lantern of this type are: 1. The lantern must be capable of giving sufficient illumination t o render the image plainly visible in moderate daylight. One of the objections to the lantern as ordinarily used for lecture purposes is that nearly complete darkness is necessary. In the ordinary teaching period, the opening and closing of shades or turning lights on or off consumes time a t the expense of instruction. I t is possible by using a short focus lens with sufficient illumination to avoid this difficulty. 2. The lantern and screen should be so placed that (a) it neither oh3tructs the student's view of the hlackhoard or apparatus used in the course of ordinary instruction, (6)the lantern can be easily and conveniently operated by the instructor. The screen should also be within the instructor's easy reach. Ease of operation is secured hy the use of a short focus lantern, as it does not require the assistance of the lantern operator. 3. The screen should he opaque to avoid undue loss of light; should he light in color: and should he capable of takina chalk which can in turn be readily removed. This last point the writer considers to be one of the most important advantages of this method of instruction. The teacher of the less elementary branches of chemistry is not generally interested in the reproduction of pictorial scenes but of data in tabular form or in graphic form. If it is desired t o emphasize some special data or t o sketch in some alternative course for a pair of variables, etc., this can he done by marking with chalk when the image is on the screen.
A number of arrangements have been tried out, some to a greater extent than others. They are all given as possibly some one of these methods would be better suited to given local conditions than the one 'The writer has found that it saves considerable time for both instructor and students m the instruction period if the more important illustrative material for a course concerned with the examination of data is ava~lahleas lantern slides. For example, in the course of several years, over a hundred slides have been collected of data illustrating the more important facts and laws dealt with in elementary physical chemistry. ~-~ he writer helie& that the time and expense connected with this collection has heen well spent. With the advent of small "still" moving picture projectographs it might he feasible to copy data and diagrams on several cartridges. Much smaller storage space would be required than for lantern slides, hut a greater projection distance would be required than for the short focus projection lantern described in this paper.
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which the writer considers to best fit his needs. The equipment in each case was a Model B Bausch & Lomb Baloptican, fitted with a 4" lens, the illuminator being a 500-watt lamp. The experiments were carried out in a lecture room 21' X 28' and having 6 rows of seats of 9 each. Outside light came from three windows on the south side of the room. The room arrangement is shown in Figure 1. Some of the possible arrangements of the lantern and screen are shown in the various subfigures of Figure 2. The arrangement showuinFigure 2a was as follows: A section of the lecture table top, some 30 inches wide was removed from the center of the table. The porseats tion removed was then hinged to the rear edge of the table, i. e., it could be used as a cover on a box. The lantern was then secured to the bottom of this piece by boltine the back lees and the front posts to it. By returning the lecture table movable piece to its original position the lantern could be stored inside the desk when not in use. I t was FIGORE1.-SHOWING ARRANGEB~NT OP ROOM found necessary in order AND LIGHTING to secure sufficient rigidity to fasten the lantern table to the back wall by a hook when in use. The screen was a Translux screen which could be mounted on the outer edge of the opening in the lecture table by a small length of gas pipe. The screen likewise could be stored in the opening in the desk when not in use. The bellows of the lantern as thus used was not adjustable and care was necessary in mounting the lantern that the bellows extension was sufficient to bring the image into approximate focus on the screen. While this arrangement has some desirable features, i. e., easily operated by the instructor and conveniently stored, it has several defects from the standpoint of the requirements tabulated previously. The instructor
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top
FIGURE ~.-%IOWING
m
S O Y E OF THE POSSIBLE ARRANGEMENTS OF THE LANTERN AND
SCREENS
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must walk around the lecture table to be in front of the screen; the screen cuts off a considerable view of the blackboard; and lastly a transparent screen itself presents several objections. One cannot write with chalk on such a screen and further, the illumination is not altogether satisfactory. Depending upon the position of the observer, a portion of the screen is much more intensely lighted than the remainder, so that in order to see the image the observer must shift his position from time to time. It should be observed that the arrangement would be somewhat more satisfactory if the screen and lantern had been mounted a t the end of the lecture table rather than in the middle. Two other arrangements for using transparent screens are shown in Figures 2b and 26. The arrangement in Figure 26 can only be used when space behind the wall is available. An opening is cut in the wall, the lantern directed against the wall, and a mirror is placed as indicated. The image then is reflected through the opening onto the Translux screen. The proper angle for mounting the mirror depends upon the angle which the optical axis of the lantern makes with the vertical and this in turn will depend upon how high the screen is placed. It is not a difficult matter to adjust these by trial and error. Only good quality mirrors should be used. The size of the mirror will depend upon how near it is placed to the lantern; the closer to the lantern the smaller the mirror required. The arrangement shown in Figure 2c is used by Prof. R. C. Moore, Department of Geology, the University of Kansas. It is particularly adapted for showing pictorial scenes in daylight, thecinstructor being his own operator. A cabinet is built as diagrammed, which is blackened on the interior. A Translux screen is placed in front of the cabinet. The optical axis of the lantern is placed parallel to the screen and the mirror a t an angle of 4.5' to the screen. In the device as used, the mirror is 30 X 40 inches and the screen 48 X 60 inches, although this could be somewhat smaller. A small hole is cut, as indicated in the diagram, to allow.the projecting beam to strike the mirror. The lantern is placed on a cabinet which serves as the lantern slide file. A self-contained portable unit, somewhat similar in arrangement, has been designed by Dr. T.T. Quirke of the University of Illinois and has been placed on the market.= None of these arrangements, however, is satisfactory if one wishes to write or draw on the screen upon which the image is present. This requires an opaque screen capable of taking chalk, which can be easily removed. The arrangement shown in Figure 2d to 2f has been used with this purpose in mind. That shown in Figure 2d represents the simplest arrangement. The lantern is best placed a t one end of the lecture table (the darkest end is, of The Projectograph Company, Station A , Champaign, Ill.
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OCTOBSR,1929
course, best) and the image thrown directly on the blackboard. The front of the lantern must be elevated in order to get the image high enough. Due to the fact that the upper rays must travel farther than the lower ones, distortion of the image is produced. If the image is not made too large, this distortion does not greatly interfere. In the absence of any better arrangement, the projection of data and diagrams will be found workable. If i t is desired to use this method permanently, a somewhat better arrangement is shown in Figure 2e. The lantern is placed at a slight angle from the vertical, within the desk, and the top of the desk, removed and made removable. A mirror ab is placed to reflect the image upon the board.
FIGURE
~ . - S X O W ~ ARRANGEMENT NG USED BY
THE AUTHOR
The mirror should be placed a t an angle of 45' with the optic axis of the instrument. It will be found that if the lantern is exactly vertical some distortion is still produced. This can be almost eliminated by placing the lantern a t a slight angle to the vertical. The opening in the desk must be of sufficient dimension to permit the instructor to easily operate the lantern. The objections to this method are that the lantern is in a somewhat awkward position and more satisfactory screen materials are available than the blackboard. The arrangement shown in Figure 2f has been tried. The lantern is
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mounted within the desk at the angle shown and the image projected on the screen described below which is hung so that it is a t right angles to the optic axis of the lantern. This method, while again workable, is open to the objections that the lantern is not placed so that it can he most easily operated and the screen is not in the most convenient position for the instructor. The arrangement shown diagrammatically in Figure 2e and photographically in Figure 3 has been found most satisfactory for the writer's purpose. The lantern is mounted, permanently a t the rear by bolting the rear legs to brackets screwed to the table. The front of the lantern is elevated3 so that the image is raised to the desired height. The screen is placed so that it is a t right angles to the optic axis of the lantern and a t the same height as the blackboard; the position being found by trial. When the correct position is found, its position is measured and brackets of strap iron made to secure it in this position. The screen was made by taking a cheap grade of blackboard (not slate but made of pasteboard or composition board) and mounting it in a light wooden frame, the dimensions of the screen being 3 X 4 feet. The face of the screen was painted with several coats of white paint4 mixed with approximately an equal volume of ground pumice; the object of the pumice being to secure a matte and gritty surface. I t was found with this screen and an image 3' X 3' that illumination was sufficient to render it visible a t any point in the lecture room without darkening the room. In a room in which oukside light fell directly on this screen, it would he necessary to shut off this light; a point to be remembered in choosing a location for the screen. As white chalk would not be visible on this screen, colored chalk was used. The writer prefers blue chalk to other colors as it shows up somewhat better than do other colors and is removed more readily. After some use, it is necessary to clean the screen with a dampened cloth. In the arrangement as used, the lens is approximately 60 inches from the screen. It should he remarked that these various arrangements require diierent methods for inserting the slides in the lantern. A little practice will soon accustom the instructor as to the proper method for a given arrangement. Greater elevation than that furnished by the adjustable legs has been found nzcessary. Any chemist should know better than to use a lead paint.
