ON THE RELATION OF OPTICS TO PAINTING. 101 



script of the object, but a translation of his impression 

 into another scale of sensitiveness, which belongs to a 

 different degree of impressibility of the observing eye, 

 in which the organ speaks a very different dialect in 

 responding to the impressions of the outer world. 



In order to understand to what conclusions this 

 leads, I must first of all explain the law which Fechner 

 discovered for the scale of sensitiveness of the eye, 

 which is a particular case of the more general psycho- 

 physical law of the relations of the various sensuous 

 impressions to the irritations which produce them. This 

 law may be expressed as follows: Within very wide 

 limits of brightness, differences in the strength of light 

 are equally distinct or appear equal in sensation, if 

 they form an equal fraction of the total quantity of 

 light compared. Thus, for instance, differences in in- 

 tensity of one hundredth of the total amount can be 

 recognised without great trouble with very different 

 strengths of light, without exhibiting material dif- 

 ferences in the certainty and facility of the estimate, 

 whether the brightest daylight or the light of a good 

 candle be used. 



The easiest method of producing accurately mea- 

 surable differences in the brightness of two white 

 surfaces, depends on the use of rapidly rotating disks. 

 If a disk, like the adjacent one in Fig. 3, is made to 

 rotate very rapidly (that is, 20 to 30 times in a second), 



