THE SENSATION OF SIGHT. 247 



harmonies of an entire orchestra, but the optician cannot 

 directly ascertain the composition of light by means of 

 the eye ; lie must make use of the prism to decompose 

 the light for him. As soon, however, as this is done, 

 the composite character of light becomes apparent, and 

 he can then distinguish the light of separate fixed stars 

 from one another by the dark and bright lines which the 

 spectrum shows him, and can recognise what chemical 

 elements are contained in flames which are met with on 

 the earth, or even in the intense heat of the sun's at- 

 mosphere, in the fixed stars, or in the nebulae. The fact 

 that light derived from each separate source carries with 

 it certain permanent physical peculiarities is the founda- 

 tion of spectral analysis — that most brilliant discovery of 

 recent years, which has opened the extreme limits of 

 celestial space to chemical analysis. 



There is an extremely interesting and not very un- 

 common defect of sight which is known as colour-blind- 

 ness. In this condition the differences of colour are 

 reduced to a still more simple system than that described 

 above ; namely, to combinations of only two primary 

 colours. Persons so affected are called colour-blind^ 

 because they confound certain hues which appear very 

 different to ordinary eyes. At the same time they dis- 

 tinguish other colours, and that quite as accurately, or 

 even (as it seems) rather more accurately, than ordinary 

 people. They are usually ' red-blind ' ; that is to say, 

 there is no red in their system of colours, and accordingly 

 they see no difference which is produced by the addition 

 of red. All tints are for them varieties of blue and green 

 or, as they call it, yellow. Accordingly scarlet, flesh- 

 colour, white, and bluish-green appear to them to be 

 identical, or at the utmost to differ in brightness. The 

 same applies to crimson, violet, and blue, and to red, 

 orange, yellow, and green. The scarlet flowers of the 



