of tJie Eye in relation to the Perception of Distance. 53 



front lines of the crystals with ultramarine, and the back lines 

 with vermilion. Several, though not all, of these when placed 

 in the stereoscope refused to appear of their true form, the 

 blue lines, whose positions would stereographically give a 

 front presentment, retiring behind the red lines of the crystal. 

 That the mind does, however, habitually make use of the 

 dissimilarity of binocular vision is evident from the old trick 

 of looking with one eye through a short tube while attempting 

 to walk up to a suspended ribbon and cut it wdth a pair of 

 scissors. Yet persons who have the use of but one eye find 

 no difficulty m performing the task ; and, with practice, the 

 feat becomes easy if the distance be carefully estimated by the 

 sensations of ocular focus. The linear distance between the 

 two eyes, however, sets a limit to the range of habitual esti- 

 mates of distance founded on the fact of binocular dissimila- 

 rity ; and, as Mr. Ruskin has observed*, it is possible to see, 

 and to see in focus together, the extreme distance and the 

 middle distance of a landscape, though the foreground and 

 distance cannot so be seen together. 



19. I cannot here dwell upon questions involving linear 

 perspective (7i). The various considerations upon which linear 

 perspective becomes a basis for the perception of distance are 

 purely associative — mental rather than optical^ geometrical 

 rather than physical. 



20. I pass on, therefore, to aerial perspective (i); the various 

 definitions of which may be summed up in one as the expres- 

 sion of distance by colour. A distant hill looks bluer than a 

 less distant one by reason of the slight opacityof the intervening 

 atmosphere (^' sky ■'); and its blueness is in some manner, to 

 a normal eye, a far truer measure of its distance than any one 

 of the considerations yet treated of. The blueness thus exist- 

 ing in nature varies with the changing conditions of atmo- 

 sphere, and hence is liable to be fallacious when the atmo- 

 spheric conditions are unfamiliar. Thus in Smith's ' Optics ' 

 it is narrated of Bishop Berkeley that, when travelling in Italy 

 and Sicily, he thought the cities as he approached them tvro or 

 three miles too near, by reason of the unfamiliar transparency of 

 the air. 



* Mr. Riiskin advances this point in defence of the practice of land- 

 scape-painters in leaving foregrounds sketcliy and unfinished, maintaiu- 

 ing that they should exhibit '• a decisiye imperfection, a firm but partial 

 assertion of form which the eye feels indeed to be close home to it, and 

 yet cannot rest upon, nor cling to, nor entirely understand, and from 

 which it is driven away of necessity, to those parts of distance on which 

 it is intended to repose." — ' Modern Painters,' vol. i. pp. 183 and 184, 

 note. Mr. Ruskin adds that Turner was the first to introduce this treat- 

 ment into landscape art. 



