1913.] Sensations and the Theory of Forced Vibrations. 493 



In some respects the phenomenon of colour is more striking than that of 

 whiteness, and probably, if it had been possible for the spectroscope to be 

 invented before any theories on the subject were thought of, the problem 

 would have rather been to explain why, with a certain intensity of illumina- 

 tion, light of different wave-lengths should appear brilliantly and variously 

 coloured, whereas with less light or with more the colours fade. 



The existence of this optimum intensity for the excitation of the sense of 

 colour is very noticeable when calibrating an ordinary students' spectroscope 

 with the Fraunhofer lines by direct sunlight. The act of measuring produces 

 sufficient fatigue to make the spectrum appear pale. If now the instrument 

 is directed towards the sky or a cloud, the colours instantly become rich and 

 brilliant, though it may be necessary to open the slit wider before there is 

 light enough to see them. 



I propose to show how this entire range of phenomena — from the scarcely 

 visible band by feeble light to the brilliantly coloured spectrum with 

 optimum intensity, and the washed-out colour with bright sunlight — is in 

 strict accordance with the laws of forced vibrations. 



It is, of course, understood that any hypothesis as to the manner in which 

 the ethereal vibrations we think of as light give rise to the sensations we 

 know as light would naturally be expressed, in the first place, in terms of the 

 electromagnetic theory. Inasmuch, however, as the same laws apply to all 

 kinds of harmonic oscillations, whether electrical or mechanical, it will be 

 more convenient to retain the phraseology proper to the mechanical model. 



Evidently, the action, whatever it be, must take place through those 

 processes which we commonly regard as chemical — processes, namely, in 

 which some rearrangement of atoms, either within the molecule or from some 

 other molecule, is brought about. 



There is a very clear statement by Kuhne of the Opto-chemical Hypothesis 

 in Hermann's ' Handbuch der Physiologie,' vol. 3, p. 327. According to him, 

 the opto-chemical hypothesis regards the visual cells as carriers of chemically 

 decomposable materials called visual substances, which, however, have no 

 effect upon the visual cells as long as they are undecomposed. But the 

 hypothesis ascribes to the decomposition products resulting from the action 

 of light on these substances the power of chemically exciting the protoplasm 

 of the visual cells. 



This excitation might conceivably result from the act of decomposition by 

 light of the visual substances, but inasmuch as the effects do not instantly 

 cease with the removal of the stimulus, it would appear that the cause is to 

 be sought rather in the material action of the decomposition products. 



In applying to this theory the elementary principles of chemical dynamics, 



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