and an Optical IllusiofU' ' • 197 



into one, when three images only were in view. The same 

 experiment may be made with two letters, or any other figures 

 or objects which are equal in size and form, as follows: — 

 J* A A Natural single vision. 



'2. AA A A View with axes slightly converged. 



f View with greater convergence of the op- 



3. A A A< tical axes and the two intermeUJatp 



[_ images coalesced into one. j-ya^K/ 



On suddenly closing either eye, this middle or superimposed 

 image did not disappear, and it was evidently made of two 

 images from two objects formed on corresponding parts of the 

 retinaj. Hence we have the converse of the case of double 

 vision of a single object, for two objects are made to produce 

 a single impression. Thus far I had proceeded in 1816, when 

 I read a paper on this subject to a club of my fellow- students 

 at Yale. *^; 



Experiment III. — In 1843 I made the experiment of cori- 

 verging the optical axes upon two contiguous figures on the 

 wall-paper of my office, in the same manner as I had done 

 with the two images of the two candles in Exp. II. When I 

 had thus succeeded in taking up optically the two figures and 

 superimposing them one upon the other, suddenly the whole 

 wail appeared to leap out from a distance of ten feet to within 

 half a yard of my eyes, where it remained in miniature beauty 

 as palpable to vision as it had been in its original place. To 

 this image, suspended as it were between the observer and the 

 object, 1 shall in the subsequent part of my paper apply the 

 term illusive image. 



It then appeared that the right eye was directed to the left 

 one of two contiguous figures, and the left eye to the right 

 figure, which, being identical in form and size, gave the im- 

 pression of a single object at the point of intersection of the 

 optical axes. Here we have two triangles formed by the two 

 optical axes intersecting each other and joined at their ex- 

 tremes; on one part by a line from one eye ''>q"«' ^"'J ^■"'-" 

 to the other, and on the other part by a line ^ ' ' 



from one figure or object to the other. These 

 last lines being parallel (see figure), where A 

 and B represent the eyes, C and D the ob- \^^ rrtt ^c\i U 

 jects, or two figures on the wall, AD the axis 

 of the eye A, BC the axis of the eye B, and 

 E the point of intersection of the axes at the 

 place of the illusive image. As these trian- 

 gles are equiangular and similar, we can de- 

 duce from them all of the equations of such 

 triangles and apply them to the optical phse- 



