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HANDBOOK OF PHOTOGRAPHY 



with air. Colored glasses are in general more satisfactory but are more expensive and 

 the sequence of the color filters becomes fixed once and for all. It is also obvious that 

 with plain white glass the three light beams cannot be of equal magnitude, but No. 1 

 beam is the strongest and No. 3 the weakest. This arrangement is satisfactory when 

 designing a color camera balanced to incandescent light. The angular position of both 



Fig. 2U. — Optical system of double-mirror camera when the mirrors are not on the same 

 vertical plane and are at an angle less than 45°. 



mirrors can be then less than 45° for evenness of light distribution as previously 

 explained. 



2. Pellicular mirrors permit any desired ratio between reflected and transmitted 

 light. The straight through or No. 3 beam is generally preferred strongest. This, 

 however, demands that greater attention be given to the question of evenness of light 

 distribution. Furthermore, the light scattered by the surface of the filters interposed 



in the path of beams 1 and 3 is directed toward 

 negative 2 with the result of a very slight fog 

 appearing in negative 2, particularly with extremelj' 

 intense light and short exposure. This results in an 

 apparently lower gamma which is in general not seri- 

 ous but should be avoided. The effect of scattered 

 light is materially reduced b}^ making the reflection 

 of the second mirror greater than its transmission 

 or by adopting the arrangement of Fig. 20. The 

 arrangement of Fig. 19 is also free from this defect, 

 but greater emphasis must be given to the problem 

 of evenness of light distribution, unless the lens is 

 of exceptionally long focus. In general the dispos- 

 ition of Fig. 20 can be considered as the most satis- 

 factory with any type of mirrors. 

 In considering the subject of scattered light and the path which it follows it is 

 solely necessary to compute its amount, which ordinarily is of the order of 41^2 per cent 

 of the incident light for normal incidence, and to consider that the partial mirrors 

 behave in identical fashion with respect to the light which arrives from the lens or 

 from a different point within the camera. 



In order to achieve the greatest possible speed, the first mirror is sometimes made 

 only verj- lightly coated, and the negative material facing beam 1 is a plain blue- 

 sensitive emulsion of very high speed exposed without blue filter. Beam 2 from the 



Fig. 21. — Double-mirror color 

 camera with mirrors crossed at 

 45°. 



