172 PROJECTION OF IMAGES OF OPAQUE OBJECTS [Cn. VII 



of the lantern slide and the reflecting qualities of the opaque object 

 (see 274a). 



275. Aperture of the projection objective for transparencies 

 and for opaque objects. By comparing figures 90-91 it will be 

 seen that for a transparency, relatively small aperture for the 

 projection objective is sufficient. This also shows that if one were 

 to use the same objective for both transparencies and for opaque 

 objects, that the difference in brightness would be enormously 

 exaggerated, if one used only the necessary aperture for the trans- 

 parencies. If one used the proper objective for the opaque object, 

 it would answer well for the transparency, but only a part of the 

 aperture would be utilized. As the large aperture makes the 

 objective very expensive, one wastes money by having the large 

 aperture for transparencies In the best practice, an objective of 

 moderate aperture is used for transparencies, and one of relatively 

 very large aperture for opaque projection. 



276. As will be shown later (Ch. XIV, 8 5 ya), with a given 

 object and a given illumination, the brilliancy of the screen image 

 depends upon the aperture of the objective and its distance from 

 the screen. The larger the diameter of the lenses of an objective 



274a. Light flux getting through the objective with opaque projection. 



It will be shown in 857a that the light received from a perfectly white, per- 

 fectly diffusing surface is 



> 



vSin 26 d20 _ _ T B 



(i cos 20) 



o 



(i cos 20) lumens per square centimeter of the white reflecting 



surface, where I is the intensity of illumination of the surface measured in 

 meter candles, and 6 is the half angle of light subtended by the objective, or 26 

 is the angle of light subtended by the objective. The light received by the 

 surface is I/io,ooo lumens and the proportion of light received by the surface 



i cos 2B 

 which strikes the objective is then 



In this problem the angle of light subtended by the objective is 20, i. e. 

 26 = 20. The proportion of light received by the objective is then (i cos 

 20)/2 = (i -9397)/2 = .0603/2 = .0302 or about T, r " ( . 



