Mixture of Prismatic Colours. 119 



and acosi^Trw'/, are mixed together, then the resulting light 

 will be represented by a cos ^icnt +■ a cos 2 tt n' t, which is 

 equal to 



{n'—n^\ n'+n, , , 



2 a cos — - — ^ > cos — — — t (1.) 



Suppose now that the two component colours are the mean 

 yellow and blue of the spectrum, then 



n — 535,000000,000000, 

 ri = 622,000000,000000 ; 



therefore ~~^ = 578,500000,000000, 



^^^ = 4-3,500000,000000. 



The numerical value of — - — here obtained is as nearly as 



possible equal to the number of undulations in a second 

 which constitutes the mean green light of the spectrum ; and 



the value of — - — is about the tenth part of the number of 



undulations in a second which constitutes the extreme red. 



Hence the formula (1.) represents a light whose colour is 

 green, but whosip intensity is variable (for we may regard the 

 whole factor in brackets as the square root of the intensity), 

 the variation being periodical, and going through all its values 

 43,500000,000000 times in one second. 



33. Now it is an experimental fact that, when the intensity 

 of light suffers a regular periodical change of any kind, the 

 eye does not perceive that change, if it take place with suffi- 

 cient rapidity; the eye being perfectly insensible to rapid and 

 regular variations in the intensity of light, on account of the 

 duration of impressions on the retina. We must, of course, 

 except the case where the periodic time of the variation is 

 equal to that of the undulations which constitute any colour 

 of the spectrum ; for, in such a case, we have every reason to 

 suppose that the variation would produce the sensation of 

 colour. [In the analogous case of sound, it is well known that 

 if any sound be repeated regularly, at intervals equal to the 

 period of vibration of any particular musical note, the sensa- 

 tion of that note will be produced.] It is likely, also, that 

 if the number of oscillations (so to speak) in a second of the 

 intensity of the light be not much less than 4-58,000000,000000 

 (the number of undulations for extreme red), or not much 

 greater than 727,000000,000000 (the number of undulations 

 for extreme violet), then, though the variation will not pro- 

 duce a distinct sensation of colour, it will not be wholly un- 



