IN SINGLE AND BINOCULAR VISION. 36:3 



The height MN of the cone, fig. 18, is =col ^ A-cot\A', A,A' being the an- 

 gles of the optic axes LMB, LNR, and OL or OR radius. But as these angles are 

 not known, we may find MN thus : Let D= distance OP ; d=8s, the distance of 

 the two points united at M ; e?,=SY, the distance of the two points united at N ; 



C=LR=2i inches. Then MP=,< NP=~~,; and MN=-^. When the two 



- 



figures are united by converging the axes beyond P, the base mn of the line will 

 be nearest the eye ; and, consequently, the cone will appear hollow. In this case, 



M'N' = - - ^ - ; and the cone will be much larger than in the other case. If 

 v/ & o d 



we make 



D = 9.24 inches, 

 C = 2.50 ; then 

 d = 2.42 ; 

 d' = 2-14; and 



MN = 0.283, the height of the cone. Whereas, in the se- 

 cond case, M'N = 18.9 feet ! 



Considering that the summit-plane op rises above the base m n, in conse- 

 quence of the convergency of the optic axes at N, it may be asked, how it happens 

 that the frustum still appears a solid, and the plane op, where it is, when the op- 

 tic .axes are converged to another point M, so as to see the base m n distinctly ? 

 Should not the relief disappear, when the condition on which it depends is not 

 fulfilled ? But, instead of the relief disappearing, the summit-plane op maintains 

 its position there as fixedly as if it belonged to the real solid ; and it ought to do 

 so, for the rays emanate from it in exactly the same manner, and form identi- 

 cally the same image on the retina as if it were a real solid. Now, by the mere 

 advance of the intersection of the optic axes from M to N, the rays from the 

 circles AB, CD, &c. still produce the same picture on the retina of each eye, and 

 the only effect of the advance of the point of convergence from N to M, is to throw 

 that picture a little to the right side of the optic axis of the left eye, and a little 

 to the left of the optic axis of the right eye ; so that the summit op still retains its 

 place, and is merely seen double. 



6. On ike Doctrine of Corresponding Points. 



Our celebrated countryman, Dr REID, calls those points in the retina of each 

 eye corresponding, which are similarly situated with respect to the foramen centrale, 

 or centre of each retina; and he maintains that objects painted on those points have 

 the same visible position. He observes " that the most plausible attempts to 



