The educational screen (c1922-c1956])

412 The Educational Scret Lantern and Slide Conducted by Dr. Carlos E. Cummings The editor of this department will attempt to answer all queries submitted, on the making or projection of lantern slides, lanterns or still projectors, or pictures made by photography for educational purposes. All matters connected with moving picture projection or films will be discussed on another page. All readers of the Educational Screen are invited to make use of this page, and submit questions on any topic properly considered herein. Converting an Old Lantern Into a New One WE have received many queries as to the possibility of installing incandescent bulbs in old types of lanterns. Most of the machines used five years ago and previously were built to be used with the electric arc or the calcium light, and as both of these types require skilled attention to operate it is very often advisable to replace this source of illumination with the bulb. The latest type of bulb is small, tubular in shape, and the filament in the form of a flat grid, making its installation much more simple than the previous types in which the bulb was round and the filament in the form of a cluster. Optically there is no reason why an incandescent bulb cannot be used in any lantern which shows a satisfactory picture with other forms of illumination. We have converted a number of calcium machines by removing the jet and screwing the socket for the lamp in its place. In this way the light can be raised and lowered or moved from side to side to get the necessary centering. In some machines the lamp housing is small and will not permit the bulb to be centered, but this is not apt to be the case in using the small tubular type of lamp. Ample ventilation must be provided as otherwise the bulb will become so hot as to melt the glass. A very important feature to be considered where conversion is attempted is the size and diameter of the projection lense. This should never be less than 2^ inches in diameter. It is unfortunate that certain dealers are turning out a machine with a projection lense of the small size, as the results obtained are not to be compared with the larger size, and the saving in cost is so slight as to be negligible. Where an incandescent bulb is placed in a lantern, particularly if the lantern is to be carried from one room to another, the cord should be attached firmly with tape or a clamp to the base of the lantern. Many of the manufacturers are very careless in this matter, bringing the cord directly to the Ian: socket, which results in the breakage of the wii close to the socket. The weight and strain of tl cord should never be allowed to fall on the sock( itself. Many of the old type lanterns were equippe with lamp houses which were open at the bad and where bulbs are installed, the back shoul be closed in order to prevent the light froi shining into the eyes of those in the audienc sitting behind the lantern. This may readily h done with a piece of stove pipe iron, or if th is not practicable the same result can be accorr plished by hanging a felt curtain on a loop c wire. Where the flat grid lamp is used, the be5 results are obtained when the plane of the gri is parallel with the surface of the condense] and as the relative position of the screw and th grid may vary with different bulbs, provisio should be made by which the socket can be turnc and clamped at the proper point. A very useful addition is the concave mirro back of the bulb. The dealers have in stock on type of bulb in which the mirror is attached to ; collar fitting on the base of the lamp, while ii other types of lantern the mirror is separate an( fastened to the back of the pedestal. The mirro should be so adjusted that its light is projecte< on the same axis as the lamp itself. This can h accomplished by placing a piece of white papei in front of the projection lense. By moving th( bellows in and out a point can usually be foun( where the image of the filament shows plainly ci the paper, and if the lamp is properly adjustec it will come in the center of the illuminated spot A misplaced mirror will show a secondary image sometimes at a considerable distance from th( image of the filament, and where this occurs tli( mirror should be adjusted until the two practically coincide. While it is true that satisfact.^'x projecticn may be secured without accurate c