498 



Dr. G. J. Burch. On Light- 



[Apr. 19, 



store, n, of visual substances to collect, but because the metabolism is so 

 active that the rate of supply dn/dt of the visual substance is greater than in 

 most people. In quite a number the rate of supply of the visual substance 

 for the blue is so great that it is not recognised as a separate sensation, but 

 confused with violet. 



According to this hypothesis, the store n of molecules of the visual 

 substance would be governed by an equation of equilibrium between the 

 causes tending to use, destroy, or disperse the exciting molecules, and those 

 by which they are formed, so that it would have a definite value for every 

 constant intensity of illumination and of physiological condition. In ordinary 

 dark-adaptation we should therefore have a large accumulation n of molecules 

 of the visual substance, so that on coming into a quite moderate light the 

 rate of formation of exciting molecules dx/dt may for a short time be greatly 

 in • excess of its ordinary value for that degree of illumination, producing a 

 sensation vividly described by the French word dblouissement* 



This gradually passes off as the store % falls to its equilibrium value for 

 that rate of demand dx/dt, when the eye reaches its new condition of 

 adaptation, the curves of the colour-sensations changing into those belonging 

 to the coefficient of friction proper to the concentration represented by the 

 new value of n. 



As the intensity of the light increases the equilibrium value of n grows 

 less — the resonance becomes more free and the curves grow peaked, so that a 

 condition is reached represented by fig. 5. Each colour is brilliantly 

 distinct and separated from the rest, dominating its own region of the 

 ■spectrum. This is the optimum intensity for the excitation of the sense 

 of colour. What happens when the light is still brighter will be described 

 in the next section. 



A BC D i F GHK 



Fig. 5. 



Resonance Curves of the Colour Sensations Bed, Green, Blue, and Violet, with the 

 coefficient of friction, k = 1 5. 



Before passing on, it must, however, be noted that according to the opto- 

 chemical hypothesis we must imagine the decomposition products to be formed 

 by the action of the light, independently of whether or when they are used. 

 * See also, for the physiological aspect of this, p. 501. 



