584 



ON THE MECHANISM OF THE EYE, 



(Plate 9. Fig. 75.) and the distance of this 

 intersection from the retina was 637 thou- 

 sandths. This nearly corresponds with tlie 

 former calculation ; nor can the distance of 

 the centre of the optic nerve from the point 

 of most perfect vision be, on any supposition, 

 much less than that which is here assigned. 

 And., in the eyes of quadrupeds, the most 

 strongly painted part of the choroid is further 

 from the neiVe than the real axis of the eye, 



I have endeavoured to express, in four 

 figures, the form of every part of" my eye, as 

 nearly as I have been able 19 ascertain it ; 

 the first (Plate 1 1. Fig. 81 ) is a vertical sec- 

 tion ; the second (.Fig. 82.) a horizontal sec- 

 tion ; the third and fourth are front views, in 

 different states of the pupil. (Fig. 83 and 84.) 



Considering how little inconvenience is 

 experienced from so material an inequality in 

 the refraction of the lens, as I liave described, 

 we have no reason to expect a very accurate 

 provision for correcting the aberration of the 

 lateral rays. But, as far as can be ascer- 

 tained by the optometer, the aberration 

 arising from figure is completely corrected; 

 since four or more images of the same line 

 appear to meet exuctly in the same point, 

 which they would not do if the lateral ray.s 

 were materially more refracted than the 

 rays' near the axis. The figure of the sur- 

 faces is sometimes, and perhaps always, 

 more or less hyperbolical* or elliptical : in 

 the interior laminae indeed, the solid angle of 

 the margin is somewhat rounded off; but the 

 weaker refractive power of the external parts 

 must greatly tend to correct the aberration, 

 arising from the too great curvature towards 

 the margin of the disc. Had the refractive 

 power been uniform, it might have collected 

 the lateral rays of a direct pencil nearly as 



• Petit. Mem. del'Acad. 172s. 20. 



well ; but it would have been less adapted to 

 oblique pencils of rays: and the eye must 

 also have been encumbered with a mass of 

 much greater density than is now required, 

 even for the central parts ; and, if the whole 

 lens had been smaller, it would also liave ad- 

 miltod too little light. It is possible too, that 

 Mr. Ramsden's observation-^-, on the advan- 

 tage of having no reflecting surface, may be 

 well founded : but it has not been demon- 

 strated, that less light is lost in passing" 

 through a medium of variable density, than 

 in a sudden transition frona one part of that 

 medium to another ; although such a con- 

 clusion may certainly be inferred, from the 

 only hypothesis which affords an explanation 

 of the cause of a partial reflection in any 

 case. But neither this gradation, nor any 

 other provision, has the effect- of ren- 

 dering the eye perfectly achromatic. Dr. 

 Jurin had remarked this, long ago;}:, from 

 observing the colour bordering the image of 

 an object seen indistinctly, Dr, W oliaston 

 pointed out to me, on the optometer, the red 

 and blue appearance of the opposite inter- 

 nal angles of the crossing lines; and men- 

 tioned, at the same time, a very elegant ex- 

 periment for proving the dispersive power of 

 the eye. He looks through a prism at a 

 small lucid point, which of course becomes 

 a linear spectrum. But the eye cannot so 

 adapt itself as to make the whole spectrum 

 appear a line ; for, if the focus be adapted to 

 collect the red rays to a point, the blue will 

 be too much refracted, and expand into a 

 surface ; and the reverse will happen if the 

 eye be adapted to the bliie rays; so that, in 

 either case, the line will be seen as a tri- 

 angular space. The observation is confirmed, 

 by placing a small concave speculum in dif- 



t Phil. Trans. 1795. a. J Smith, e. 90. 



