344 PHYSIOLOGICAL PHYSICS. [Chap. xxvn. 



moderately the fibres sensitive to red and to green, 

 and feebly the fibres sensitive to violet, the resultant 

 sensation being yellow. So the vibrations corre- 

 sponding to the blue of the spectrum excite moderately 

 the fibres sensitive to green and violet, and feebly 

 those sensitive to red, the resultant sensation being 

 blue. Thus the sensations of various kinds of colour 

 are all due to excitations in different degrees of three 

 sets of nerve-fibres in the retina, each set being 

 specially affected by vibrations of definite rapidity. 



Qualities of compound colour. Tone of 

 colour is determined by the simple colour which pre- 

 dominates in the mixture. Intensity is dependent on 

 the amplitude or extent of the vibratory movements 

 of the ether by which the sensation of light is produced. 

 The degree of saturation of colour signifies the extent 

 to which the colour is or is not mixed with white 

 light. The colours of the spectrum are fully 

 saturated. 



CHAPTER XXVII. 



ABERRATIONS OF LENSES. 



Chromatic aberration. A lens is practically 

 an arrangement of prisms. Thus, a doubly convex 

 lens may be considered as two prisms set with their 

 bases together, the angles being rounded off, as shown 

 in Fig. 154, and a doubly concave lens may be con- 

 sidered as two prisms with their edges together. 

 Now we have seen, in the last chapter, that when a 

 ray of white light passes through a prism it is dis- 

 persed into its seven constituent colours, red, orange, 

 yellow, green, blue, indigo, and violet, because of the 



