198 Mr. J. Locke oft Single and Double Vision^ 



nomena. Thus the distance from the eye to the illusive image 

 ( AE) will be to the distance from the object to the same image 

 (DE) as the distance between the eyes (AB) is to the distance 

 between the objects (CD) the figures or pannels on the paper, 

 &c. 



It is not merely the two objects directly in the axes of the 

 eyes which coincide, but every contiguous pair of objects seen 

 obliquely will also coincide, and form the illusive picture in 

 extenso. Indeed the optical operation of convergence seems 

 like taking up a duplicate copy of the figures lyingin the first 

 place exactly over them, and slipping it gradually to the ex- 

 tent of one figure, until again the figures coincide in a new 

 place. 



Some of the phenomena of the Illusive Image. — It is quite 

 perfect, and can be viewed deliberately and critically as if it 

 were a real substance in place as it appears; the figures are 

 smaller than the originals in proportion as they are nearer; 

 as the outlines are a little blended by double pictures not ex- 

 actly coincident, an elegant softening and a playful beauty 

 exalts their effect above that of the original ; as the head 

 moves sideways, upward or downward, the illusive image 

 moves, but with a diminished motion ; as the head is inclined 

 to the right or left, the superimposed pictures slide out from 

 each other, the one ascending and the other descending to the 

 extent of the inclination. 



Optical Equivalency. — The illusive image and the erroneous 

 distance at which it appears, show evidently that philosophi- 

 cally we do not see an object, but we contemplate an image on 

 the retina. If this image can by any means be formed without 

 the object, we still contemplate the substance such as would 

 produce that image. Thus in Exp. III., and the figure illus- 

 trating that experiment, the two objects C and D produce 

 each picture in the eyes at A and B, exactly as would be pro- 

 duced by a single object of smaller size at E. Thus the two 

 objects, one at C and the other at D, " fulfill the conditions 

 of the problem" of the images on the retina, exactly as it 

 would be fulfilled by a single smaller object at E. In both 

 cases identical pictures are formed on "corresponding" parts 

 of the two retinae. Hence the two objects produce the im- 

 pression of a single image. 



Directions how to make the experiment of the Illusive Image. 

 — With two identical objects only, although it is easy to su- 

 perimpose them as in Exp. II., yet the illusion of distance 

 can scarcely be attained. But with a papered wall having a 

 repetition of the same figure at equal distances, a person who 

 has voluntary command of the optical axes will soon move the 

 double images to coincide, when presently the illusion will be 



