6; 8 THE SENSES 



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 refracting power, either of the cornea 

 or of the lens; or 2, by changing the position of the lens relative to the retina, 

 as in the focusing 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 re- 

 quired in even the widest range of vision is extremely small, for from the re- 

 fractive powers of the media of the eye the 

 difference between 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 required for vision at 

 all distances, supposing the cornea and lens 

 to remain the same, would not be more than 



about 2.5 mm. Beer has shown that the 

 FIG. 457. Diagram Showing . . 



Three Reflections of a Candle, second method is indeed the type of accom- 



i, From the anterior surface of modative apparatus in fishes. But in man 



cornea; 2, from the anterior . 



surface of lens; 3, from the an d the higher animals accommodation occurs 



posterior surface of lens. For by the first method, i.e.. by changing the con- 

 further explanation, see text. . ... . 



The experiment is best per- vexity of the refracting surface. 



formed by employing an in- The accommodation of the human eye for 



strument invented by Helm- . . ./ 



holtz, termed a Phakoscope. 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, figure 457. The first and brightest is, i, a small erect image 

 formed by the anterior convex surface of the cornea; the second, 2, is also erect, 

 but larger and less distinct than the preceding, and is formed at the anterior 

 convex surface of the lens; the third, 3, is smaller, inverted, and indistinct; 

 it is formed at the posterior surface of the lens, which is concave forward, and 

 therefore, like all concave mirrors, gives an inverted image. If, now, the eye 

 under observation be made to look at a near object, the second image be- 

 comes smaller, clearer, and approaches the first. If the eye be now adjusted 

 for a far point, the second image enlarges again, becomes less distinct, and 

 recedes from the first. In both cases alike the first and third images remain 

 unaltered in size, distinctness, and relative position. This proves that during 

 accommodation for near objects the curvature of the cornea, and that of the 



