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direction, of a kind dependent on their relative densities and 

 their curves. Luminous pencils coming from remote objects 

 consist of parallel rays, and having regard to the small open- 

 ing of the pupil, the jiencils which enter the eye from an 

 object twenty or more feet distant, may be considered to be 

 composed of parallel rays. Now the refractive power of the 

 eye is such, that parallel rays entering it are collected in exact 

 foci upon its retina, without the exercise of any vital effort, 

 the eye itself being quite passive. It would occur as well 

 in a dead eye, so long as its media remained transparent, and 

 while they retained their proper curves. The luminous 

 pencils which a near object sends to a spectator's eye consist 

 of divergent rays, and the unaided refractive power of the 

 eye which sufficed to unite the parallel rays from a distant 

 object in the bacillary plane of the retina is insufficient to 

 collect divergent rays in exact foci on this plane. The foci 

 of those rays lie behind the retina, which the pencil strike 

 as spots, the sections of cones, called circles of dispersion, 

 and not as points. The result of this is a blurred confused 

 image, and not a clear one, the production of which requires 

 the rays to be brought to exact foci in the bacillary layer. 



We are, however, conscious that we possess the power of 

 seeing distinctly near objects as well as distant ones, which 

 proves that the eye has the power to unite divergent as well 

 as parallel rays in exact retinal foci, and this implies the 

 possession of a power of altering its refractive state so as to 

 suit it to the distance of the object we desire to see distinctly, 

 or in other words to adapt it to the degree of divergence of 

 the luminous rays entering the eye from the object. This 

 adaptation of the refractive state of the eye is technically 

 called its accommodation. It has been at different times 

 attributed to a change of figure of the eyeball, to an altera- 

 tion of the curve of the cornea, and to a shifting of the 

 position of the lens ; but more delicate methods of observa- 

 tion than were formerly at the command of the physiologist, 

 have shown all these vicAvs to be erroneous, and by the direct 

 inspection of the human eye with instruments specially con- 

 trived for this investigation, it has been demonstrated that 

 its accommodation for a nearer object is effected by increased 

 convexity of the lens, and this chiefly of its anterior surface 

 — the curve of its posterior surface being altered in a scarcely 

 appreciable degree. With the increased convexity of the 

 lens, its axis is proportionately lengthened, the pupillary 

 region of the iris approaches the cornea, and the peripheral 

 portion of the iris recedes from it. There are not any 

 grounds for supposing that this change of figure is wrought 



