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HANDBOOK OF PHOTOGRAPHY 



3. In case of a partial mirror made of plain white glass aluminized and camera 

 Imlanced for incandescent and photoflood light, the mirror must be metal coated to a 

 ratio of reflected to transmitted light of 3:1. By making the reflected beam the 

 greater of the two, the secondary image created by the back surface of the mirror is 

 not suflicientlj^ strong to record. With this arrangement the two front emulsions of 

 the tripack are exposed at right angles in the reflected beam without filter, and the 

 third or back emulsion of the tripack in the transmitted beam through the standard 

 red filter (Wratten No. 25). The ground glass for focusing can be placed in the 

 reflected beam. The glass mirror, of course, must be optically flat but not too thick, 

 otherwise the distoi'tion caused by the refraction of the glass might become too great. 

 A moderate amount of distortion can be compensated by tilting the plateholder 

 carrying the red negative. 



4. To avoid the refraction error and the presence of any secondary image, the 

 partial mirror can be made of extremely thin transparent material, such as collodion, 

 stretched on an optically flat frame. This transparent film can be also coated with a 

 thin deposit of evaporated gold or aluminum to the desired reflection-transmission 

 ratio. In this case it is immaterial as to which of the two beams is the greater. 

 Obviouslj' the bipack will be exposed in the path of the stronger beam. The idea of 

 using pellicular mirrors for color cameras is quite old (see L. Geisler, U. S. Pat. 1060444, 

 Apr. 29, 1913). Geisler also suggested coloring the thin transparent mirrors for the 

 purpose of using them for filters as well. 



When the reflected beam of a pellicular mirror is much lower than the transmitted 

 beam, then the angle of the mirror with respect to the optical axis should be preferably 



less than 45° unless a very long focal-length 

 lens is used, otherwise there is a danger of 

 uneven distribution of light throughout the 

 area of the reflected beam. This becomes 

 immediately evident from Fig. 13. 



The angles of the boundary rays in Fig. 



4- 



4 









10 



Fig. 14. — Uneven angle of reflection 

 of boundary rays for mirror placed at 

 45°. 



20 30 40 50 60 

 Angle of Incidence 

 Fig. 15. — Reflection coefficient of a glass 

 surface in air as function of angle of incidence. 

 Index of glass = 1.55. 



14 are, respectively, 35° and 55°. The coefficient of reflection for a pellicular sur- 

 face having an index of reflection of 1.55 would be 5 and 9.5 per cent, respectively, 

 for the surface without any metallic coating. The unevenness of illumination is there- 

 fore quite apparent. This unevenness, however, decreases materially as the thick- 

 ness of the metallic coating increases. 



The construction of this type of camera is quite simple as the only necessary 

 requirement is to maintain the length of the optical paths identical with respect to 

 both beams. 



Double-mirror Camera. — The most satisfactory type of camera for three-color- 

 separation negatives is the double-mirror type. The mirrors in this camera can be 



