



On Single and Double Vision, and an Optical Illusion. 71 



J liquely 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 lying in the first place exactly 

 over them, and slipping it gradually to the extent of one figure, 

 until again the figures coincide in a new place. 



Sojrie 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 exactly coincident, an ele- 

 gant softening and a playful beauty exalts their effect above that 

 of the original ; as the head moves sideways, upward or down- 

 ward, the illusive image moves, but with a diminished motion ; 

 as the head is inclined to the right or left, the superimposed pic- 

 tures 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 philosophically 

 we do not see an object, but we contemplate an image on the retina, 

 II this image can by any means be formed without the object, 

 we still contemplate the substance such as would produce that 

 image. Thus in Experiment III, and the figure illustrating that 

 experiment, the two objects C and D produce each picture in the 

 eyes at A and B, exactly as would be produced by a single ob- 

 ject 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 im- 

 ages 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 retinas. Hence the two 

 objects produce the impression of a single image. 



Directions how to make the experiment of the Illusive Image. 



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

 impose them as in Experiment II, yet the illusion of distance can 

 scarcely be attained. But with a papered wall having a repeti- 

 tion of the same figure at equal distances, a person who has vol- 

 untary command of the optical axes will soon move the double 

 images to coincide, when presently the illusion will be perfect. 

 Persons who have not this command of their eyes may succeed 

 in obtaining the proper convergence by looking at a finger held 

 about fifteen inches from the face, while standing ten feet from 

 a wall with figures twenty inches apart. 



Apparent distances of objects. — It seems that we judge of 

 moderate distances by a kind of triangulation, the distance be- 

 tween the eyes being a constant base line. In order to put this 

 to the test, 1 have several times made the actual measurements as 

 in the following cases : 



