CH. xxxin. VELOCITY OF LIGHT. 329 



different sensations would overlap and blend to pioduce 

 the gradual tints of the spectrum. This theory has now 

 been most perfectly worked out by Professor Helmholtz, 

 and the diagram, Fig. 60 a, shows by three curves the 

 amount of excitement supposed to be produced on the 

 three kinds of nerve-fibre. Thus the curve R shows how 

 the left end of the spectrum affects the red nerve-fibres 

 most strongly, while yet some effect is produced for almost 

 the whole length ; while in the same way the green and 

 violet nerve-fibres have each their own maximum of excite- 

 ment. An equal stimulation of all the fibres, such as is 

 produced by an ordinary ray of light, gives a sensation of 

 white. This explanation was most curiously confirmed by 

 results obtained by Clerk-Maxwell in colour-blindness, for 

 he showed that it is exactly one of these three primary 

 colours, generally red, that people affected with colour- 

 blindness are unable to distinguish. They see only two 

 colours and their combinations, which suggests that the 

 nerve -fibres answering to the third colour are weak or 

 deficient 



Chief Works consulted. Young's ' Lectures on Natural Philosophy,' 

 1845; Peacock's 'Life and Works of Young;' Arago's ' Eloge of 

 Fresnel ; ' Herschel's ' Lectures on Familiar Subjects ; ' Tyndall, ' On 

 Light ; ' Spottiswoode's ' Polarization of Light ; ' Whewell's * Inductive 

 Sciences ; ' Helmholtz, ' Physiologische Optik ' and Popular Scientific 

 Lectures, * 



