COLOR PHOTOGRAPHY 



627 



arranged in various manners, each of which offers its own advantages and disad\an- 

 tages. In Figs. 16 to 21 are shown the arrangements most commonly used. 



For convenience of reference the three partial beams are indicated throngliout as 

 No. 1 for the first reflection, No. 2 for the second reflection, and No. 3 for the beam 

 straight through. 



Fig. 16. — Optical system 

 of two-mirror camera with 

 mirrors at right angles. 



Fig. 17. — Optical system of 

 two-mirror camera in which 

 first mirror is at 45° angle and 

 second at smaller angle to inci- 

 cent light. ", 



There is indeed very little difference between the various arrangements and each 

 one can be properly set up to give satisfactorj^ performance, provided that the necessary 

 precautions are taken in carrying out each design. In addition it might be said that in 

 general the weak points of one particular arrangement are not necessarily shared by 

 the others. 



Fig. 18. — Optical system of 

 double-mirror camera in which 

 both mirrors are at angles 

 smaller than 45° to the incident 

 light. 



Fig. 19. — Optical system 

 of double-mirror camera in 

 which both mirrors are 

 parallel and at 45° to the 

 incident beam. 



The following general remarks apply to all the various arrangements: 

 1. Partial mirrors of plain white optical glass should have the reflected beam 

 gi'eater than, or at least equal to the transmitted beam. This is necessary to reduce 

 the secondary image to a negligible quantity imless the back surface is evaporated 

 with a transparent substance of refractive index lower than that of the glass itself, 

 or unless the surface of the glass is chemically treated to reduce its index in contact 



