174 PROJECTION OF IMAGES OF OPAQUE OBJECTS [Cn. VII 



277. Brilliant screen images of opaque objects. It is intel- 

 ligible from the above discussion and the diagrams that to produce 

 a brilliant screen image of an opaque object five things are neces- 

 sary: 



1. The light for illuminating the opaque object must be very 



brilliant, like sunlight or the electric arc light. 



2. The opaque object must be capable of reflecting most of the 



light illuminating it, or must be on a white background. 



3. The projection objective must have lenses of large diameter. 



4. The distance of the objective from the screen must not be 



too great. 



5. Besides the above, the projection room must be dark or the 



screen image will not have sufficient contrast. 



FIG. 94. DOLBEAR'S OPAQUE PROJECTOR WITH SUNLIGHT. 



(From Dolbear's Art of Projecting). 



H Heliostat, porte-lumiere or simply a plane mirror to direct the sunlight 

 through the bi-convex condenser. 



r Movable mirror to reflect the sunlight upon the opaque object at d. 

 The handle for changing the inclination of the mirror is seen at the right. 



d Opaque object with the light from the mirror (r) illuminating it. 



o Projection objective. 



S Screen for the image. 



278. Position of the radiant. The radiant or source of light 

 for illuminating opaque objects for projection may have either of 

 two positions: 



