CONTRIBUTIONS TO SCIENCE. 475 



neutral line for its whole length (see Plate II.) The posi- 

 tion corresponding to the pure sensation which is missing 

 in the colour-blind is near the red, but outside the triangle, 

 and hence corresponds to a purer red than any in the solar \1 

 spectrum (see p. 480). 



The colour-blind generally class all tints as yellows or J||| 

 blues. According to Maxwell's theory, their sensations cor- 

 respond to green and blue ; but the reason why they regard ( 

 yellow as brighter than green lies in the fact that although , } 

 to the normal eye the sensation of yellow is a combination 

 of the sensations of red and green, yet yellows are so much 

 brighter than greens that light from them generally excites 

 the green sensation more powerfully than that from green , 

 objects themselves, and hence yellows are more conspicuous 

 than greens to the colour-blind, but only in virtue of the / 

 green they contain. 



To enable colour-blind persons to distinguish between 

 red and green, Maxwell had a pair of spectacles constructed, 

 one eye-glass of which was red and the other green, so that 

 the object appeared differently to the two eyes. These 

 spectacles cause objects to appear to possess a metallic lustre 

 on account of the different ways in which they are presented 

 to the eyes. To the colour-blind a red object would appear 

 brighter when seen through the red glass, while a green 

 object would appear brighter through the green glass. 



There are colour-blind persons whose vision is very 

 different from that described by Maxwell in his early 

 papers, but to these we shall again refer presently 

 (p. 482).. 



It would occupy too much space to describe the differ- 

 ent methods adopted by Maxwell for the purpose of mixing 

 light of different colours, but his colour box demands a 

 special notice, both on account of the perfection with which 

 it is adapted to the object in view and the extreme beauty 

 of its arrangement. The first two boxes constructed were of 

 inconveniently large dimensions. The form we shall de- 

 scribe is that finally adopted, and the shell, as well as the 

 principal optical apparatus of the box, are still at the 



