492 



Dr. G. J. Burch. On Light- 



[Apr. 19, 



Pig. 2 



End Elevation of Oscillating Bar A, showing its connection, by the link-work, E, F, G, H, 

 with the Pendulums C and D, the two Eesonators in tune with which are oscillating 

 violently, and the rest scarcely at all. 



The periodic variations of amplitude in the response which die out after 

 the impressed force has been in action for a little while are easily seen, and, 

 in fact, the apparatus affords an excellent illustration of §47 and §48 in 

 Volume 1 of Eayleigh ' On Sound.' 



F or my present purpose the main interest of the apparatus lies in the 

 possibility of utilising it to elucidate one of the most difficult problems of 

 colour- vision— the problem, namely, of white light. 



Newton's discovery of the physical fact that the prism separates the beam 

 into rays of different refrangibilities and different colours affords no explana- 

 tion of the physiological fact that any one of these colours, even the most 

 brilliant, should disappear with absolute completeness in presence of the 

 others. Hering's theory of the antagonism of red and green, and of blue and 

 yellow, is, from this point of view, a most natural one. It would be simple 

 enough, on Young's theory, to explain why all colours tend to white by very 

 strong light, for each of his three hypothetical " nerves " is assumed to be 

 affected, though to a different degree, by light of all wave-lengths, so that we 

 have only to suppose that, with a sufficiently strong stimulus, all three 

 " nerves " are almost equally excited. But this leaves us without any valid 

 explanation of that other fact, that by very feeble light all colours tend 

 to grey. 



