t)0 On the C/iro)nafi'c AlhTratuvi of tJw I\i/t\ 



scene by colour, not. by retinal niaonitiules or binoeularly 

 conipound sensations. 



42. I have endeavonred to ai)[)ly the nietliod of experi- 

 mentation in anotlier direction. Take any picture in wliich 

 there are well-expressed t'on^gronnds and distances, and look 

 at it through thie coloured media employed in the earlier 

 experiments. If the facts of distance are expressed only 

 indirectly, by suggestions of magnitude, hy linear perspective 

 of vanishing lines and the like, the interposition of a red or a 

 blue glass ^^ ill produce little effect, except that the latter may 

 d(\ulcn the intcMisity of the shadoAvs. If, however, the ex- 

 pression oi' distance in the picture is accomplislied chiefly 

 by aerial pcrspectiv(^ (that is to say, by colour), a red glass 

 will almost destroy the intelligibility of the picture, while a 

 blue glass will draw out the dislnncos in a marked manner. 



4o. 1-Jefiecting how useful is tlie purpose subserved thus 

 by the non-achromatism of the eye, i consider it probable that 

 if the eye were so constructed as to be originally achromatic, 

 having usually blue distances and red-brown foregrounds to 

 look at, it would, by an inevitable process of natural selection, 

 develop into a non-achromatic instrument. 



Sii))i))i(U\i/ a/hl /■xei'opifiihifio?}. 



44. 1 would conclude therefore : — 



(a) That tlie muscular sensation of the adjustment to focus 

 of the lenses of the eye a fiords a possible means of estimating 

 distances. 



(^/S) That when binocular nu^thods, and those depending on 

 association of visible form or ma-gnitude fail, the eye falls 

 back on colour as a means oi' ostiniating distance. 



{^y) That estimates of disi-Aucc IoihuUhI on apparent mag- 

 nitude are liable to be rendered fallacious by the colour of 

 the object. 



(S) That, conversely, estimates of distance founded on 

 colour are liable to be contused In' apparent magnitude. 



(e) That colour may in some cases outweigh, as a criterion, 

 the evidence of binocular vision. 



(f) That the chromatic aberration of the eye accounts for 

 the universal opinion of painters as to the ^' retiring *' character 

 of blue, and the " advancing *' character of red tints. 



(?/) That Aerial rerspeciive in Art is a true expression of a 

 physical fact in the perception of distance. 



Universitv Colleo-e, l^ri^tel, 

 MuN- 8. 1877. 



