104 ON THE KELATION OF OPTICS TO PAINTING. 



difference between daylight, on the one hand, and day- 

 light plus candle-light on the other, can be no longer 

 recognised. 



This law is of great importance in discriminating 

 between various degrees of brightness of natural objects, 

 A white body appears white because it reflects a large 

 fraction, and a grey body appears grey because it re- 

 flects a small fraction, of incident light. For different 

 intensities of illumination, the difference of brightness 

 between the two will always correspond to the same frac- 

 tion of their total brightness, and hence will be equally 

 perceptible to our eyes, provided we do not approach too 

 near to the upper or the lower limit of the brightness, 

 for which Fechner's law no longer holds. Hence, on 

 the whole, the painter can produce what appears an equal 

 difference for the spectator of his picture, notwithstand- 

 ing the varying strength of light in the gallery, provided 

 he gives to his colours the same ratio of brightness as 

 that which actually exists. 



For, in fact, in looking at natural objects, the abso- 

 lute brightness in which they appear to the eye varies 

 within very wide limits, according to the intensity of 

 the light, and the sensitiveness of the eye. That which 

 is constant is only the ratio of the brightness in which 

 surfaces of various depth of colour appear to us when 

 lighted to the same amount. But this ratio of bright- 

 ness is for us the perception, from w T hich we form our 



