1874.] Colour-combinations by Polarized Light. 357 



Effect of Combinations of two Colours. A similar train of reasoning 

 might be applied to the triple overlaps. But the main interest of these 

 parts of the figure consists in this, that each of the triple overlaps is 

 complementary to the fourth single image, since the recombination of 

 all four must reproduce white light : hence the tint of each triple over- 

 lap is the same to the eye as the mixture of the two tints suppressed in 

 the remaining image ; and since by suitably turning the Nicol 1ST or the 

 prism Pj, or both, we can give any required position to the two bands of 

 extinction, we have the means of exhibiting to the eye the result of the 

 mixture of the tints due to any two bands at pleasure. 



Effect of Combinations of three Colours. A further step may be made in 

 the combination of colours by using a third quartz, Q 2 , and a third double- 

 image prism, P 2 , which will give rise to eight images ; and if C C' be 

 the complementaries extinguished by the combination Q 2 P 2 , the formula 

 for the eight images may be thus written : 



OOO C+B+A. 



OOE C+B' + A'. 



OEO C' + B'+A. 



OEE C' + B + A'. 



EOO C' + B + A. 



EOE C'+B' + A'. 



EEO C+B' + A. 



EEE C+B+A'. 



The total number of combinations of tint given by the compartments 

 of the complete figure will be : 



Q 



- =8 single images. 



Q tj 



Y" 2 = 28 overlaps of two. 



- 8.7.6 



three. 



four. 



five. 



six. 



seven, 



eight. 

 Total 255 



