50 Mr. S. P. Thompson on the Chromatic Aherration 



})erception of want of i)Ositionai adjustment wlien the corre- 

 sponding parts of the two images do not fall npon the corre- 

 sponding points of the two retinae. 



7 (d). I(etinal magnitude of an object as appreciated by 

 the muscular effort of turning the eyeball so as to bring the 

 parts of the image successively into the centre of the field 

 of vision. 



8 (^). lletinal magnitude as a sensation of excitement of a 

 certain area of nerve matter of the retina. 



9 (/). The muscular sensation of effort of the ciliary 

 nmscle in the adjustment of the eye to exact focus. The 

 perception of exactness or inexactness of focus which guides 

 the muscular action is, too, an optical effect in itself of little 

 Aalue as a means of estimating distance, except in so far as 

 it controls the required muscular contraction. 



10 {g). Binocular dissimilarity of the images on the t\^o 

 retinas, which, conspiring to give one mental impression, pre- 

 sent slight differences that suggest the idea of distance. 



11 (A). Linear perspective. Under this term I propose to 

 include, beside the usually understood })lay of lines and 

 diminishing apparent magnitudes commonly spoken of as 

 perspective, such artificial means of estimating distance as 

 are afforded by comparison with objects of known size 

 (figures, cattle, &c.), and such notions of position, surface, &c. 

 as can be drawn from comparison with houses or other objects 

 of known form and magnitude. 



12. (i) Aerial Perspective, in its widest sense. On this 

 point Helmholtz has most aptly remarked* that the brain 

 has the same means for the appreciation of relative distance 

 as are employed by the painter on his canvas; and he enume- 

 rates amongst these means strong shadows, bold relief in the 

 foreground, obscurity of atmosphere, and dimness of outline, 

 in addition to contrasts with objects of known size or form. 

 The term, however, is often restricted to the peculiar altera- 

 tion of the colour of objects due to the intervening atmo- 

 sphere f. 



II. Relative Importance of foregoing Data. 



13. It is obvious that the considerations advanced in (a), 

 (Z>), and (c) are foreign to the present inquiry, though of 



* 'Popular Lectures,' Englisli edition, p. 281. 



t Mr, Euskin's definition of aerial perspective is, from an artistic point 

 of view, of value. "Aerial perspective is tlie expression of space by any 

 means vrliatsoever, sliarpuess of edge, vividness of colour, kc. assisted by 

 greater pitch of shadow, and requires only that objects should be detached 

 Irom each other by degrees of intensity m propo7iion to their distance.' — 

 'Modern Painters,' vol. i. p. ISO. 



