358 On Colour-combinations by Polarized Light. [May 21, 



The most interesting features of the figure consist in this, that the 

 subjoined pairs are complementary to one another, viz. : 



OOO EOE 



C+B+A C'+B' + A' 



EDO OOE 



C' + B+A C+B' + A' 



EEO OEE 



C+B'+A C' + B+A' 



EEE OEO 



C+B+A' C' + B' + A 



And if the prisms P, P x , P 2 are so arranged that the separations due to 

 them respectively are directed parallel to the sides of an equilateral tri- 

 angle, the images will be disposed thus : 



OEO OOO 



EEO EOO OEE OOE 

 EEE EOE 



The complementary pairs can then be read off, two horizontally and 

 two vertically, by taking alternate pairs, one in each of the two vertical, 

 and two in the one horizontal row; and each image will then repre- 

 sent the mixture of the three tints suppressed in the complementary 

 image. 



Low-tint Colours. A slight modification of the arrangement above de- 

 scribed furnishes an illustration of the conclusions stated by Helmholtz, 

 viz. that the low-tint colours (couleurs degradees), such as russet, brown, 

 olive-green, peacock-blue, &c., are the result of relatively low illumina- 

 tion. He mentioned that he obtained these effects by diminishing the 

 intensity of the light in the colours to be examined, and by, at the same 

 time, maintaining a brilliantly illuminated patch in an adjoining part of 

 the field of view. If therefore we use the combination N, Q, P, P 1 (i. e. 

 if we remove the second quartz plate), we can, by turning the prism P 

 round, diminish to any required extent the intensity of the light in 

 one pair of the complementary images, and at the same time increase 

 that in the other pair. This is equivalent to the conditions of Helm- 

 fa oltz's experiments ; and the tints in question will be found to be 

 produced. 



