358 Sir David Brewster on the Law of Visible Position 



within the range of our eyes, do not appear sensibly greater 

 than they are in reality, it follows, that the visible point which 

 sends a ray to the cornea, is seen sensibly in its place, and 

 consequently in the direction of a line joining the point itself 

 and its image on the retina. But why," D'Alembert adds, 

 " is this the case ? It is a fact which I will not undertake to 

 explain*." 



When we consider the data from which D'Alembert has 

 deduced the preceding results, it is not easy to account for his 

 having abandoned the inquiry as a hopeless one. He employs 

 the dimensions of the eye as given by Petit and Jurin, and he 

 assumes Jurin's index of refraction for the human crystalline 

 lens, though it is almost exactly the same as that of an ox, as 

 given by Hawksbee. These, indeed, were the best data he 

 could procure; but he should have inquired if the most pro- 

 bable law of visible direction was compatible with any other di- 

 mensions of the eye, and any other refractive powers of the 

 humours which were within the limits of probability; and 

 above all, he ought to have examined experimentally the truth 

 of his fundamental assumption, that visible points are really 

 seen in their true places when they are not in the axis of vision. 



Now it is quite certain that these points are not seen in their 

 true direction, and that there is an ocular parallax, which is 

 the measure of the deviation of the visible from the true direc- 

 tion of objects. This parallax is nothing in the axis of the 

 eye, and it increases as the visible point is more and more 

 distant from that axis; and hence it follows, that, during the 

 motion of the eyeball, when the head is immoveable, visible 

 objects not only change their place, but also their form. 



Had the eye consisted of only two concentric coats, a cornea 

 and a retina, filled with a homogeneous fluid, vision would 

 have been performed by centrical pencils ; — the visible and 

 the true direction of points would have coincided, and objects 

 would have changed neither their form nor their position du- 

 ring the motion of this hypothetical eyeball round the common 

 centre of the two coats. But as such an eye could not have 

 afforded sufficiently distinct vision, the introduction of the 

 crystalline lens became necessary ; and it is owing to the se- 

 condary refractions at its surfaces and within its mass of vari- 

 able density, that the parallax of visible direction is produced. 



The following experiment will establish the existence, and 

 explain the nature of this parallax. Let M N, fig. 1, be the 

 eyeball, C the centre of curvature of the retina, and also the 

 centre of motion of the eyeball. Having placed an opake 

 screen S several inches from the eye, till its inner edge just 

 * Opuscules Mathematiques, torn. i. mem. ix. p. 266. 



