CH. VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS 175 



1. It may be in front of the object so that the light emitted 



shines directly on it. This is the original device and gives 

 the greatest amount of light (fig. 92-93); or the radiant 

 may be tilted (fig. 105, in). 



2. The second method is to have the light not in front, but a 



mirror reflects the light from the radiant upon the opaque 

 object (fig. 94, 95). This is usually a more convenient 

 arrangement than the above, but a certain amount of the 

 light (between 10% and 25%) is lost when reflected 

 from a mirror. 



279. Use of a condenser or concave reflector with opaque 

 projection. This is frequently employed for the object is often at a 

 considerable distance from the radiant, and too small a part of the 

 light from the radiant would be available but for the help of the 

 condenser. 



In most cases only the first element of the condenser is used. 

 This projects upon the object or the mirror a cylinder of parallel 

 rays (fig. 90,103). Sometimes also a converging lens of long focus 

 is put in the path of the parallel cylinder to concentrate it more or 

 less, depending upon the size of the object to be shown. Instead 

 of a condenser, there is sometimes used a reflector (fig. 95, 96) 

 behind the radiant. 



280. Darkness of the projection room. Owing to the diffi- 

 culty of obtaining a sufficiently brilliant screen image it is necessary 

 to have the projection room very dark. 



COMBINATION LANTERN SLIDE AND OPAQUE PROJECTION 

 281. Daylight and twilight vision. Nearly all modern 

 apparatus giving opaque projection also gives transparency pro- 

 jection with a slight change. These two kinds of projection are 

 mutually antagonistic for the adjustments of the eyes of the specta- 

 tors. For transparency projection the image is so brilliant that 

 the eyes are adjusted for daylight vision in large part, while for 

 the opaque projection the image is so dim that the eyes should be 

 adjusted for twilight or night vision. 



