CH. VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS 171 



face instead of traversing it, that is, it must extend in the opposite 

 direction from that used with the transparency. 



The light falling upon the face of the opaque object must then 

 be reflected from each point. But unlike the tranteparent object, 

 in which practically all of the light illuminating each point of the 

 object goes directly to the projection objective (fig. 91), with the 

 opaque object, each point reflects the light irregularly and in all 

 directions within the entire hemisphere ( 1 80 degrees, fig. 90) . This 

 being the case, only a part of the light reflected from each point can 

 get into the projection objective, all the rest falling outside the 

 objective. Of course, the larger and closer the objective, the more 

 of the light will be received; hence, in selecting an objective for 

 opaque projection, keep in mind that the greater the diameter of 

 the lenses the more light from each point can be received, and con- 

 sequently the more brilliant will be the screen picture. 



It is assumed in this discussion, and in the accompanying dia- 

 grams (fig. 90-91), that the opaque object is black and white and 

 that it and the transparent lantern slide are of the same size; 

 that both are lighted by a similar beam of parallel light rays, and 

 that none of the light is lost by absorption. 



274. Relative amount of light for the images with trans- 

 parencies and opaque objects. If, for example, as in the diagram, 

 the projection objective can receive but 20 degrees of the hem- 

 isphere of light from each point, then 160 degrees will fall outside the 

 objective and not aid at all in the formation of the screen image. 

 If the objective could take in all of the light from each point, the 

 opaque object would give as brilliant a screen image as the lantern 

 slide, but the actual proportion of light represented by the angle of 

 twenty degrees is only three per cent, of that represented by 180 

 degrees. As only three per cent, of the light from each point helps 

 in the formation of the screen image of the opaque object, the 

 opaque object can give a screen picture only three per cent, as 

 bright as the transparency where practically all of the light helps 

 to form the screen image (fig. 90-91). 



In practice, how great a proportion of light serves for the screen 

 image and how much is absorbed or lost depends upon the opacity 



