PHYSIOLOGY OF VISION EYE ACHROMATIC. 239 



In the prosecution of his experiments in some of which he was 

 assisted by M. Biot M. Magendie found, as might have been expected, 

 that any alteration in the relative proportion or situation of the dif- 

 ferent humours had a manifest effect upon vision. When a minute 

 opening was made in the transparent cornea, and a small quantity of 

 the aqueous humour permitted to escape, the image had no longer the 

 same distinctness. The same thing occurred when a little of the 

 vitreous humour was discharged by a small incision made through the 

 sclerotica. He farther found, that the size of the image on the retina 

 was proportionate to the distance of the object from the eye. When 

 the whole of the aqueous humour was evacuated, the image seemed to 

 occupy a greater space on the retina, and to be less distinct and lumi- 

 nous; and the removal of the cornea was attended with similar results. 

 When the crystalline was either depressed or extracted, as in the ope- 

 ration for cataract, the image was still formed at the bottom of the 

 eye; but it was badly denned; slightly illuminated, and at least four 

 times the usual size. Lastly, when the cornea, aqueous humour, and 

 crystalline were removed, leaving only the capsule of the crystalline 

 and the vitreous humour, an image was no longer formed upon the 

 retina : the light from the luminous body reached it, but it assumed no 

 shape similar to that of the body from which it emanated. 



Most of the results as M. Magendie 1 remarks accord well with 

 the theory of vision. Not so, the distinctness of the image under these 

 deranging circumstances. According to the commonly received notions 

 on this subject, it is necessary, in order to have the object depicted with 

 distinctness on the retina, that the eye should accommodate itself to 

 the distance at which the object is placed. This is a subject, however, 

 that will be discussed presently. 



Such are the general considerations relating to the progress of 

 luminous rays from an object through the dioptrical part of the organ 

 of sight to the nervous portion the retina. We shall now inquire into 

 the offices executed by such of the separate parts that enter into its 

 composition as have not already engaged attention. 



We have shown, that the cornea, aqueous humour, crystalline, and 

 vitreous humour, are a series of refractive bodies, to concentrate the 

 luminous rays on the retina ; to keep the parietes of the eye distended ; 

 and to afford surface for the expansion of the retina; thus enlarging 

 the field of vision. It is probably owing to their different refractive 

 powers, that the eye is achromatic; or, in other words, that the rays, 

 impinging upon the retina, are not decomposed into their constituent 

 colours, an inconvenience which appertains to the common lens (Fig. 

 82). The eye is strictly achromatic; and it has been an object of 

 earnest inquiry amongst philosophers to determine how the aberration 

 of refrangibility is corrected in it. Euler, 2 first perhaps, asserted, that 

 it is owing to the different refractive powers of the humours; and he 

 conceived, that, by imitating this structure in the fabrication of lenses, 



1 Precis Elementaire, i. 73. 



a Mem. Berlin, p. 279, pour 1747; and Letters of Euler, by Sir D. Brewster, Amer. 

 edit., i. 163, New York, 1833. 



