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diameter can be obtained. This is due to the increase of the angle at the projection lens subtended by the him at these small distances. Accordingly, the curves f are the highest ones of Fig. 11 that can now be considered as of interest commercially. Development along the line of lenses of smaller f : value is recommended as offering the best chance at present, for increase of illumination in this type of projection.
In these experiments the 4.5-inch condenser has been used as a measure of quantity of illumination only, in order to connect the light given by the smaller condensers with thai louna previously with the plano-convex, meniscus and prismatic. It is, of course, impossible, as there shown, to use the plano-convex at the position of maximum illumination, as given in Figures 10 and 11, when using a source of ununiform brilliancy, such as an incandescent filament. If this 4.5-inch plano-convex is thrown so far out of the position of maximum light that it will give a screen uniform to the eye, then, according to the previous findings, we cannot get an illumination greater than about one-half that of curve a for a .5 inch source, by the use of any of the projection lenses now avail ble. i his gives, then, a practical value of C equal to .Ov30 for the 4.5 plano-convex, using a standard aperture, a .5 inch light source, and a 2.5 inch projection lens of 5.5-inch focal length.
The small condensers, on the ether hand, give an approximately uniform field at their positions of maximum brightness. However, for sources larger than .3 inches on a side, there are at present no lamps available in wdiich the wall of the lamp bulb is so close to the filament that it can be brought to w^ithin that distance from the optical center of the condensers wdiich gives the maximum illumination. There is another practical limitation. AMien using sources as high as .3 inches on a side, in a small bulb, the brightness of the source cannot be made as great as in the larger bulb lamps, if the life of the lamp is to be the same in the two cases. The greatest illumination thru a standard film aperture that can be obtained with the small condensers, is, then, perhaps .055, .05, and .047 respectively — and this with an expenditure of energy of but little more than one-third that required to give the .030 with the 4.5 inch plano-convex.
In the paper to which reference has already been made, one of the prismatic condensers (No. 3) gave 4 per cent more light than the plano-convex we are now using as a standard of reference ; and, what is more important, at this point of maximum illumination the screen was uniformly lighted. Adding this 4 per cent to the maximum value of C for the large condenser of Figure 10, we get .062 as the maximum useful light obtainable with the prismatic condenser. It will be noticed, however, that for the small projection lenses this superiority of the prismatic over the small condensers is inappreciable.
It is possible that more light than this .055, .05, and .047 could be obtained by the use of the .5 inches source further from the condenser. However, since the .5-inch source of the 900-watt lamp sub
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