150 Royal Society : — 



8 



- =8 single images^ 



8 7 



t-~q *■ 28 overlaps of two. 



8.7.6 

 1.2.3 



8.7.6.5 

 1.2.3.4 



8.7.6 



= 56 „ three. 



== 70 „ four. 



= 56 „ five. 



= 28 „ six. 



seven. 



1.2.3 



8.7 

 1.2 



8 



1 



1 =1 „ eight. 



Total 255 



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' 



EOO 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 1 ,P 2 are so arranged that the separations 

 due to them respectively are directed parallel to the sides of an 

 equilateral triangle, 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 represent the mixture of the three tints suppressed in 

 the complementary image. 



Low-tint Colours. — A slight modification of the arrangement 

 above described furnishes an illustration of the conclusions stated 



