in Single and Binocular Vision, 449 



D as 9*24 inches, 

 C = 2-50; then 

 rf=2-14; 

 d! = 2*42 ; and 

 M N = 0-283, the height of the cone. 

 Whereas, in the second case, M'N= 18*9 feet! When 



C = d, M P = 5^ an d M' F infinite. 



Considering that the summit-plane op rises above the base 

 m n, in consequence of the convergency of the optic axes at 

 N, it may be asked, how it happens that the frustum still ap- 

 pears a solid, and the plane op, where it is, when the optic 

 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 identically 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 A B, 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 the Doctrine of Corresponding Points. 



Our celebrated countryman, Dr. Reid, calls those points in 

 the retina of each eye corresponding, which are similarly situ- 

 ated 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 account for this property of the eyes 

 have been unsuccessful, and that it must be either a primary 

 law of our constitution, or the consequence of some more ge- 

 neral law which is not yet discovered." This doctrine has 

 been very generally admitted ; and if great names could have 

 given it currency, those of Newton and Wollaston, supported 

 by a number of anatomists and metaphysicians, might have 

 placed it, both optically and metaphysically, beyond the reach 

 of challenge. The doctrine of the semi-decussation of the 

 fibres of the optic nerve, as explained by Newton, gave great 

 support to the theory of corresponding points. The idea that 

 each fibre of the nerve divided itself into two, one of which 



Phil. Mag. S. 3. Vol. 24. No. 161. June 1844. 2 G 



