of the Eye in relation to the Perception of Dista7ice. 49 



entering the eye, do not all come to focus upon the retina at 

 once, but the t'oci for ditferently coloured rays are differently 

 situated with respect to the retina. 



Hence, so far as the perception of the distance of a luminous 

 object is dependent upon the accurate focusing of rays upon 

 the retina of the eye, so far the want of adjustment in the eye 

 introduces a soarce of error into the data of our perception. 

 If the eye be not achromatic, a blue object and a red object of 

 equal dimensions placed side by side cannot be in focus at 

 once, and hence must appear either unequally distant or else 

 of unequal magnitude. 



2. The object of this paper is to demonstrate that the fact 

 thus briefly sketched out affords a true and legitimate explana- 

 tion of certain empirical rules of artistic practice relative to 

 the pictorial expression of distance. The existence and uni- 

 versal recognition of such empirical rules has presented itself 

 as a problem to the mind of the writer for the past six years ; 

 but it is only within the last few months that the solution now 

 offered has been adopted by him. 



It is proposed therefore : — 



I. To enumerate the various data dependent on the eye and 

 not upon the feet or other limbs, for the formation of a per- 

 ception of distance. 



II. To discuss the weight to be attached to the several data 

 under various circumstances. 



III. To inquire how far these data may be dependent upon 

 the apparent colour of an object, or upon the formation of an 

 exact focus upon the retina of the eye. 



lY. To give a brief account of some experimental results, 

 together with conclusions derived therefrom. 



I. Data for the Perception of Distance. 



3. The following nine data may be enumerated as contri- 

 buting to the formation of a perception of distance. 



4 (a). Apparent motion of objects caused by motion of the 

 observer. If we move from one place to another, or alter 

 the position of the head, the visible configuration of objects 

 suffers a corresponding change, which by association with 

 previous experience suggests an external system, of whose dis- 

 tance this " parallax " afibrds us an idea. 



5 (^). Apparent change of configuration of objects accord- 

 ing as they are viewed with one eye or the other. This affords 

 a ''parallax" similarly to (a), though on a more limited scale. 



6 (c). The muscular sensation of convergence or divergence 

 of the two eyes when directed towards an object. This is a 

 genuine muscular perception, though guided by the optical 



PA'i/. Mag. S. 5. Vol. 4. No. 22. July 1877. E 



