352 SIR DAVID BREWSTER ON THE LAW OF VISIBLE POSITION 



to which the cornea and the retina are concentric, it is obvious that if there was 

 no crystalline lens, pencils, incident perpendicularly on the cornea, would pass 

 through the common centre, and fall perpendicularly upon the retina. Hence, in 

 this case, the line of risible direction would coincide with the line of real direc- 

 tion, and also with the incident and intromitted ray. Now, the refractions at the 

 crystalline are exceedingly small, and, at moderate inclinations to the axis, the 

 deviations from the preceding law are very minute. At an inclination of 25 or 

 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. The deviation, of course, increases with the inclination ; 

 but as there is no such thing as distinct vision out of the axis, and as the indis- 

 tinctness increases with the inclination, it is impossible to ascertain, by ordi- 

 nary observation, that any deviation exists. Hence the mechanical principle of 

 D'ALEMBERT, which he himself has rejected, and the law of visible direction, 

 which I have established, are substantially true. As the Almighty has not made 

 the eye achromatic, because it was unnecessary, so He has, in the same wise eco- 

 nomy of His power, not given it the property of seeing visible points in their real 

 direction. 



Had it been necessary to make the visible ray coincident in direction with 

 the incident ray, it might have been effected by giving such a form and variable 

 density to the crystalline lens as to make the ray which it refracted cross the axis 

 of vision at the centre of curvature of the retina ; and if the crystalline lens were 

 such that this crossing point was variable, this variation might have been compen- 

 sated by making the retina spheroidal, with a variable centre of curvature. 



That a visible point is seen in the direction of a line perpendicular to the sur- 

 face of the retina at which the image of the point is formed, may be established ex- 

 perimentally in the following manner. Having expanded the pupil by belladonna, 

 look directly at a point in the axis of the eye. Its image will be formed by a cone 

 of rays variously inclined from 85 to 90 to the surface of the retina. While the 

 point is distinctly seen, intercept all these different rays in succession, and it will 

 be found that each ray gives vision in the same direction, the visible point retain- 

 ing its position. Hence it follows, that on the part of the retina in the axis of 

 vision, all rays, however obliquely incident, give the same visible direction per- 

 pendicular to the surface of the membrane. That the same property is possessed 

 by every other part of the retina cannot be doubted, and may be proved by direct 

 experiment. 



Although D'ALEMBERT states it as unquestionable, that when the visual ray 

 is in the axis of vision, or the optic axis, and passes to the retina without refrac- 

 tion, the point which emits it will be seen in the direction of a line passing from 

 its image to the visible point ; yet, after he has found that his mechanical prin- 



