228 HERMANN VON HELMHOLTZ 



have an entirely new sensation that is otherwise wanting. But 

 if the over-tones are absent in a melodic interval, we have no 

 new perception ; the only result is that a part of the sensation to 

 which we are accustomed in greater or less intensity, which 

 makes us more certain about the magnitude of the interval than 

 our memory of it, is now wanting. But nothing new or 

 unfamiliar appears in its stead. I might rather compare it with 

 the binocular vision of an object, and that of a picture. The 

 former, like a melody with over-tones, gives sensational data, 

 which enable us to judge very definitely of the dimensions of 

 depth ; the picture, which does not give these, is like the melody 

 without over-tones; but if we know the object well, we can form 

 a lively conception of it, and under many conditions it is really 

 hard to determine without direct experiment whether binocular 

 vision actually assists our perception of depth or no. The 

 essential point seems to me to be, that melody is the image of a 

 movement, and that it is possible to measure the intervals by 

 direct sense-perception. If we are able from memory to recog- 

 nize any given interval, then in particular cases we can forgo 

 the standards of measurement, without being altogether astray, 

 even if the impression of the melody takes on somewhat of the 

 weakness of the memory-image. On the other hand, I must say 

 from my own experience, that tones with unharmonious partials 

 (unless these be very weak, or very remote from the over-tone), 

 give quite false melodies, which, however, can be recognized in 

 memory as copies of the true melody. The principle you 

 require in order to obviate the undifferentiated fusion of the 

 over-tones, and also to give the relation of tones in melody, is, 

 I think, provided by the fact (or hypothesis) that tones of 

 different pitch affect different nerve-fibres.' 



Helmholtz's researches in physiological optics were only 

 interrupted for a very short time by his work on the formation 

 of ice and glaciers. On February 24, 1865, he gave a lecture to 

 the Nat. Hist. Med. Verein at Heidelberg, 'On some Properties 

 of Ice/ in which he discussed the origin of the phenomenon 

 known as the regelation of ice, while in the same month in 

 a popular lecture ' Ice and Glaciers ', which opened with a 

 brilliant description of the glacier world, he went more closely 

 into the question then so much discussed, of the movement of 

 glaciers. 



