180 



COLOR PHOTOGRAPHY. 



VI. — The Prism Experiment. 



The prism was so placed upon one-half of the photographed spectrum 

 that the line between the hypotenuse and the side face I (fig. 1) cut 

 similar color-lines at right angles. The eye of the observer was placed 

 in the prolongation of the same surface I (the arrow indicates the line 

 of vision) so that a line S drawn, before the experiment, in the direction 

 of a single color, as, for example, the yellow appeared straight when 

 viewed through the air and prism. 



It can then be calculated what change the color at the line S under 

 the prisin must experience as compared with that in the air if the 

 colored image is caused by stationary lightwaves. 



That color appears in general whose wave length is equal to the 

 difference in path between two rays reflected at two adjacent elemen- 

 tary mirrors. 



Fig. 1. 



Fig. 2. 



Let I and II in fig. 2 be two such elementary mirrors at a distance d 

 apart in a medium of an index of refraction n 2 . The rays are incident 

 in the general case at the angle a in an equilateral prism of index of 

 refraction n x and a base angle a 1} so that the rays pass through the 

 sides of the prism at right angles both in entering and leaving. 



The plane parallel liquid layer between prism and photographic 

 film has no influence on the difference of path between the interfering- 

 rays S x and S n , and neither do the phase-changes on reflection at I 

 and II. 



Let 2 « 2 be the excess of distance of S„ over S : in the layer, and a 2 

 the angle which S ir makes within the layer to the normal to the mirror. 



Let 2 di be the excess of S t over S„ in the prism. 



If we denote the wave length in air by A the difference in path 

 between S x and S u is then measured in wave lengths as follows: 



But 



Therefore 



D=2a, 



>h 



■la x 



a 2 = and a { = d tan a- z sin a\ 



cos a 2 



I) 



2d n- z — n\ sin <t, sin a 2 



A 



COS a % 



