360 Sir David Brewster on the Law of Visible Position 



York, give similar results. When we add to these considera- 

 tions the fact that the refractive power of the crystalline lens 

 assumed by D' Alembert is nearly triple of what it really i s, 

 we are entitled to reject the results of his calculations. 



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

 namely, that according 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 visible direction 

 would coincide with the line of real direction, 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 indistinctness increases with the inclination, it is impos- 

 sible to ascertain, by ordinary 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 ceconomy 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 varia- 

 tion might have been compensated by making the retina 

 spheroidal, with a variable centre of curvature. 



That a visible point is seen in the direction of a line per- 

 pendicular to the surface of the retina at which the image of 

 the point is formed, may be established experimentally 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, 



