THE SENSATION OF SIGHT. 217 



least for the practised ear, completely like another, composed of 

 different tones ; whereas, if the ear perceived musical tones as 

 the eye colours, every accord might be completely represented by 

 combining only three constant notes, one very low, one very 

 high, and one intermediate, simply changing the relative strength 

 of these three primary notes to produce all possible musical 

 effects. 



In reality we find that an accord only remains unchanged to 

 the ear, when the strength of each separate tone which it con- 

 tains remains unchanged. Accordingly, if we wish to describe 

 it exactly and completely, the strength of each of its component 

 tones must be exactly stated. 



In the same way, the physical nature of a particular kind of 

 light can only be fully ascertained by measuring and noting the 

 amount of light of each of the simple colours which it contains. 

 But in sunlight, in the light of most of the stars, and in flames, 

 we find a continuous transition of colours into one another 

 through numberless intermediate gradations. Accordingly, 

 we must ascertain the amount of light of an infinite number of 

 compound rays if we would arrive at an exact physical know- 

 ledge of sun or star light. In the sensations of the eye we need 

 distinguish for this purpose only the varying intensities of three 

 components. 



The practised musician is able to catch the separate notes of 

 the various instruments among the complicated harmonies of an 

 entire orchestra, but the optician cannot directly ascertain the 

 composition of light by means of the eye ; he 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 ap- 

 parent, 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, ov 

 even in the intense heat of the sun's atmosphere, 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 foundation of spectrum analysis that most brilliant dis- 



