OF THE EYE AS AN OPTICAL INSTRUMENT. 547 



the other hand, if the cornea be too flat, and the refractive power of 

 the humours be too low, the convergent rays proceeding from an object 

 at a moderate distance will not meet upon the retina, but behind it (if 

 they were allowed to pass on) ; consequently, the picture is indistinct ; and 

 it can only be made clear, either by withdrawing the object to a greater 

 distance, which will bring the focus of the eye nearer to its front, or 

 by interposing a convex lens to increase the refractive power of the eye. 

 Such a condition is termed presbyopic (from its being common in aged 

 persons), or long-sighted. It may proceed to such an extent, that not 

 even the removal of the object to any distance can permit the formation 

 of a distinct picture ; so that the assistance of a convex lens must be 

 obtained even to see remote objects clearly ; though a less degree of 

 convexity will be required, than for the clear vision of nearer objects. 

 This state is particularly well marked after the operation for cataract ; 

 for the removal of the crystalline lens so greatly diminishes the refrac- 

 tive power of the eye, as to render necessary the assistance of convex 

 lenses of high curvature. 



959. The power, by which a healthy, well-formed eye can accommodate 

 itself to the distinct vision of objects at varying distances, is a very re- 

 markable one ; and its rationale is not yet properly understood. Accord- 

 ing to the laws already stated ( 955, V. and VI.) the picture of a near 

 object can only be distinct, when formed more remotely from the lens 

 than the picture of a distant object. Consequently when the eye, that 

 has been looking at a distant object, and has seen it clearly, is turned 

 to a near object, a distinct picture of the latter cannot be formed without 

 some alteration, either in the distance between the refractive surfaces 

 and the retina, or in the curvature of the former. It seems most pro- 

 bable that, in the Human eye, this adjustment is chiefly effected by the 

 automatic contraction of the ciliary muscle ; which, by drawing the lens 

 nearer to the iris, will thus increase its distance from the retina. But 

 this may not be the sole change. 



960. The various humours and containing membranes of the Eye, 

 thus answer the purpose of a most delicate and self-adjusting Optical 



Fig. 164. 



Distribution of Capillaries in vascular layer of Retina. 



instrument ; the sole part, which is immediately concerned in the act 

 of sensation, being the Retina, or net-like expansion of the Optic nerve, 



