346 £. P. Lathrop on Single and Double Vision. 



2 at the moment there was no convergence. On the other hand, 

 if the head be inclined to the left, at the same angle, the images 

 would assume a similar position but would coalesce differently, 



as the convergence was diminished, as follows : A/ A . If the 



head, in either of these cases, be made to lie parallel to the shoul- 

 ders, or so that the eyes will be vertical, the images will assume 



1 3 



the following position: ^ ^, where the images marked 1 and 



D D 

 4 are the illusive images when the head is inclined to the right, 

 and those marked 2 and 3 the illusive when the head is inclined 

 to the left. The position of these images is found to be the same 

 as will be assumed, when the convergence of the optical axes is 

 brought to bear upon two pieces of paper, placed the one above 



the other, in the following manner : . 



Instead of the intermediate images in No. 2, in the experiment 

 with two pieces of paper, being made to coalesce with one an- 

 other, as seen in No. 3 of the same experiment, they can be made, 

 while passing each other, to turn out to the right or left, as fol- 

 lows : DDno, or O D u □ where the images move in the 



direction of the lines with which they are connected. 



In performing the third experiment of Dr. Locke, u of con- 

 verging the optical axes upon two contiguous figures on the wall 

 paper," and thereby obtain an image in front of the wall — called 

 by him " illusive image" — I was not so successful. An " illusive 

 image" is indeed seen, but it possesses so much of that character as 

 not to permit of my treating of it. I find no difficulty, however, 

 in superimposing pictures, or pieces of paper of different sizes. 



The principle involved in all of these results seems to be em- 

 braced in the 5th inference of the first experiment of Dr. Locke, 

 of seeing two images when the convergence of the optical axes 

 is brought to bear upon one object, viz., "that certain parts of 



fi 



>f 



f one eye correspond to certain specific parts of 



gle vtsion is the result 



Those corresponding parts lie inward in one eye and outward in 

 the other, viz. : both to the right or both to the left. From each 

 of those corresponding parts of the retina, it is probable that the 

 fibres of the optic nerve proceed, and severally unite at the point 

 of anatomical communication where the optic nerves cross, before 

 entering the brain; hence the single impression or single image." 



