TKANSACTIONS OF THE SECTIONS. 9 



to visible objects, different from their true position and magnitude. 

 Having removed this difficulty, I proceeded to examine the other data 

 upon which D'Alcmbert reasoned. According to the anatomy of the 

 eye which he adopted, the centre of curvature of the retina, which he 

 supposes to be spherical, (as he does the eye-ball,) is equidistant from 

 the extremity of the axis, or the foramen ovale, and the centre of the 

 crystalline lens. This, however, is far from being the case. M. Dutour, 

 M. Maurice, a recent and able writer on vision, and, which is of more 

 consequence, Dr. Thomas Young, have all made the centre of curva- 

 ture of the retina, at the bottom of the eye, coincident with the centre 

 of the spherical surface of the cornea ; and this centre, in place of being 

 almost half way between the apex of the posterior surface of the lens 

 and \kiQ, foramen ovale, is actually almost in contact with that apex. The 

 dissections of Dr. Knox, and of Mr. Clay Wallace, of New York, give 

 results conformable with those of Dr. Young ; and almost all these 

 authors regard the human eye as a spheroid. When we add to these 

 considerations the fact that the refractive power of the crystalline lens 

 assumed by D'Alembert is nearly triple of what it really is, we have no 

 scruple in concluding that the results of his calculations are inadmissible. 

 Assuming, then, the most correct anatomy of the eye, namely, that 

 according to which the cornea and the bottom of the retina have the 

 same centre of curvature, it is veiy clear that if there was no crystalline 

 lens, pencils incident perpendicularly upon the cornea will pass through 

 this common centre, and fall perpendicularly upon the retina. Hence, 

 in this case, the line of visible direction will coincide with the line of 

 real direction, and also with the incident and refracted ray, and will 

 likewise pass through the centre of curvature of the retina. Now, the 

 refractions at the surfaces of the crj^stalline are exceedingly small, and 

 at moderate inclinations to the axis the deviations from the preceding 

 law are very minute. At an inclination of 30°, a line perpendicular to 

 the point of impression on the retina passes through the common centre 

 already referred to, and does not deviate from the line of real visible 

 direction more than half a degree, a quantity too small to interfere with 

 the purposes of vision. At greater inclinations to the axis of the eye, 

 the deviation of course increases ; but as there is no such thing as di- 

 stinct vision out of the axis, and as the indistinctness increases with the 

 inclination of the incident ray, it is impossible to ascertain by ordinary 

 observation that such a deviation exists. Hence, the mechanical prin- 

 ciple of D'Alembert, and the law of Dr. Reid, are substantially true. 

 If the retina is spheroidal, the centre of visible direction will shift its 

 place along the axis of vision, and will correspond to the points where 

 lines perpendicular to the surface of the spheroid cut its lesser axis. 

 As the Almighty has not made the eye achromatic, because it was un- 

 necessary, so he has, in the same wise economy of his power, not given 

 it the property of seeing visible points in their real directions. 



