ACCOMMODATION 643 



If this focus occurs at a point either in front of or behind the retina, indis- 

 tinctness of vision ensues, in the way we have just described, with the pro- 

 duction of a halo. The focal distance, i.e., the distance of the point at 

 which the luminous rays are collected from a lens, besides being regulated 

 by the degree of convexity and density of the lens, varies with the distance 

 of the object from the lens, being greater as this is shorter, and vice versa. 

 In other words, the luminous points on the object and the focal points on the 

 retina are conjugate foci. Hence, since objects placed at various distances 

 from the eye can within certain limits be seen with almost equal distinctness, 

 there must be some provision by which the eye is enabled to adapt itself, so 

 that, at whatever distance the luminous object may be, the focal point may 

 always fall exactly upon the retina. 



Accommodation is the act of adapting the eye to vision at different distances. 

 It is obvious that the effect might be produced in either of two ways, viz., 

 i, by altering the convexity, and thus the re- 

 fracting pow r er, either of the cornea or of the 

 lens; or 2, by changing the position of the 

 lens relative to the retina, as in the focussing 

 of a camera, so that whether the object be 

 near or distant, the focal points to which the 

 rays are converged by the lens may always 

 fall exactly on the retina. The amount of 

 either of these changes which is required 

 in even the widest range of vision is ex- 

 tremely small, for from the refractive powers 

 of the media of the eye the difference be- { J^- ot 

 tween the focal distances of the images *?$?& 

 of an object at a distance and of one at 

 four inches is only about 3.5 mm. On 



this calculation the change in the distance of the retina from the lens re- 

 quired for vision at all distances, supposing the cornea and lens to remain 

 the same, would not be more than about one line. Beer has shown that the 

 second method is indeed the type of accommodative apparatus in fishes. 

 But in man and the higher animals accommodation occurs by the first 

 method, i.e., by changing the convexity of the refracting surface. 



The accommodation of the human eye for objects at different distances is 

 primarily due to a varying shape of the lens, its front surface becoming more 

 or less convex, according as the distance of the object looked at is near or 

 far. The nearer the object, the more convex the front surface of the lens, 

 up to a certain limit, and vice versa ; the back surface takes little or no share 

 in accommodation. The following simple experiment illustrates this point: 

 If a lighted candle be held a little to one side of a person's eye, an observer 

 looking at the eye from the other side sees three distinct images of the flame, 



