THE RECEIVING SCREEN. 387 



surfaces of suitable curvatures ; a combination in which this has been ef- 

 fected is termed an achromatic lens ; and if, at the same time, by proper 

 arrangements, the spherical aberration has been destroyed, the lens is 

 termed aplanatic. 



Now the aqueous humor, as bounded by the cornea in front and the 

 crystalline lens behind, acts as a convex, and therefore con- Conver ent 

 verging lens, and to this effect the crystalline itself adds pow- media of the 

 erfully, the two conjointly causing the images of external ob- eye ' 

 jects to form upon the black pigment. These images are, of course, in- 

 verted. 



The adjustment of the eye for perfect vision of objects at different dis- 

 tances is accomplished by the action of the ciliary muscle, ^d'ustment b 

 the requisite movement being to draw the lens farther from the ciliary mus- 

 the black pigment when the object is near. There has been c 

 much controversy as to the manner by which this adjustment for dis- 

 tance is effected, but it is generally now agreed that it is done in the 

 manner just mentioned. There has also been a difference of opinion as 

 respects the actual screen upon which the images form. Some of the 

 early optical writers regarded the black pigment as being The receiving 

 that receiving surface, an opinion which has been universal- screen V 3 the 

 ly abandoned, the function having been of late attributed to aMnoUhe ret- 

 the retina, but, as it appears to me, on totally insufficient ina - 

 grounds. The arguments against the retina, both optical and anatom- 

 ical, are perfectly unanswerable. During life it is a transparent medium, 

 as incapable of receiving an image as a sheet of clear glass, or the at- 

 mospheric air itself; and, as will presently be found, when we come to 

 describe its structure, its sensory surface is its exterior one, that is, the 

 one nearest to the choroid coat. But the black pigment, from its perfect 

 opacity, not only completely absorbs the rays of light, turning them, if 

 such a phrase may be used, into heat, no matter how faint they may be, 

 but also discharges the well-known duty of darkening the interior of the 

 eye, and therefore preventing indistinctness through the straying of the 

 rays of light. Perfection of vision requires that the images should form 

 on a mathematical superficies, and not in the midst of a transparent me- 

 dium. The black pigment satisfies that condition, the retina does not. 



Spherical aberration is compensated for partly by the increasing dens- 

 ity of the lens toward its centre, and partly by the action Correction for 

 of the iris, which stops such rays of light as are at any con- spherical aber- 

 siderable distance from the axis of the eye, acting in the 

 same manner as a perforated plate or diaphragm in ordinary optical in- 

 struments. 



It does not appear that there is any attempt at correcting the chromat- 

 ic aberration of the eye, though it is popularly supposed that the cornea, 



