Prof. A. 'M.' Mayer's Researches in Acoustics. 363 



sensation of the highest has vanished ; and the fundamental 

 remains — x — of a second after the cessation of the sensation of 



12 6 



the harmonic next ahove it. 



The successive rates of increase of the ordinates of the curve 

 which expresses our law, as we go from the ordinate belong- 

 ing to the highest note to that belonging to the lowest, repre- 

 sent the rate of successive extinctions of these harmonics in 

 the composite residual sensation. These successive changes in 

 timbre are well illustrated by sounding all the twenty forks of 

 the harmonic series of Ut 2 and then stopping the vibrations 

 successively, going from the highest to the lowest. 



The remarkable phenomenon we have just described has also 

 its counterpart in the analogous series of changes in visual sen- 

 sations which happen when the eye has received the sudden 

 impress of a bright white light and is then immediately closed 

 in darkness. Thus the average duration of the residual sensa- 

 tion in the eye is the J? of a second for lights of moderate 

 intensity ; but if the image of a bright cloud be received on 

 the eye for i of a second, the " positive sensation " remains for 

 twelve seconds. The duration of this residual sensation de- 

 pends on the colour — lasting longer for red than for violet, and 

 longer for violet than for green. Here an analogy with our 

 sonorous sensations is presented ; for the setherial vibrations 

 producing red are fewer in number than those producing either 

 green or violet, and the sensation of red lasts longer than either 

 green or violet ; and therefore it follows that we should have 

 the residual image of the sun go through these changes — white, 

 greenish-blue, blue, violet, purple, red ; and this is what really 

 happens when the sun's image is momentarily formed on the 

 retina, and the eye then kept in darkness. 



The above analogy, however, is imperfect if it really is estab- 

 lished that the residual sensation of violet lasts longer than 

 that of green when the vibrations giving these two colours 

 have equality of energy. The analogy also is one of sensations, 

 not one of the mechanisms existing between the agents and the 

 sensations they produce ; for, in the case of the ear, anatomical 

 facts give us bases for the explanation of the ear's power of 

 effecting a sonorous analysis, and for the understanding of the 

 reason of our law of the duration of the residual sensation. In 

 other words, in the ear we have laid before us the mechanism 

 (1) of the receiving apparatus, (2) of the transmitting appara- 

 tus, and (3) of the sensory apparatus ; but in the eye we com- 

 prehend only (1) and (2), but we know as yet nothing that gives 

 us an understanding of the dynamics of the sensory apparatus. 

 For has modern histology given us any facts concerning the 

 structure of the human retina which point to the establish- 



