312 ON LIGHT. 



experiment. If water, oil, or any other liquid be intro- 

 duced between the glasses, the rings are observed to 

 shrink in diameter, and the more so (and to the exact 

 extent required by theory) the greater the refractive 

 power of the liquid, 



(97.) If the sensation of colour be, in analogy to that 

 of tone or musical pitch, dependent on the frequency of 

 the vibrational movements conveyed to our nerves of 

 sensation, it becomes highly interesting to ascertain their 

 degree of frequency, in order to establish the relation be- 

 tween the two senses of hearing and seeing in that re- 

 spect. The ear, we know, can discriminate tones only 

 between certain limits, comprising about nine octaves, 

 the lowest sound audible as a note making about 16, 

 and the highest about 8200 vibrations per second. 

 Taking the velocity of light (as above) at 186,000 miles* 

 per second, and reckoning 33,866 wave-breadths to the 

 inch for the extreme red, 43,197 for the soda-yellow, 

 and 70,555 for the extreme violet, we find for the im- 

 pulses on the retina per second which produce these 

 sensations of colour, respectively, the following enormous 

 numbers : 



Extreme red, . . . 399,101,000,000,000 



Soda-yellow, . . . 509,069,000,000,000 



Extreme violet, . . , 831,479,000,000,000 



These extremes are nearly in the proportion of 2 to i, 

 so that the whole range of visual sensation on this view 

 of the subject is comprised in about one octave. If the 



* Roughly, looo million feet. 



