THE SENSES. 207 



since objects placed at various distances from the eye can, within a certain 

 range, different in different persons, be seen with almost equal distinct- 

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

 itself, so that whatever length the focal distance may be, the focal point 

 may always fall exactly Upon the retina. 



This power of adaptation of the eye to vision at different distance* has 

 received the most varied explanations. It is obvious that the effect might 

 be produced in either of two ways, viz., by altering the convexity or in- 

 tensity, and thus the refracting power, either of the cornea or lens; or by 

 changing the position either of the retina or of the lens, so that whether 

 the object viewed be near or distant, and the focal distance thus increased 

 or diminished, the focal point to which the rays are converged by the 

 lens may always be at the place occupied by the retina. The amount of 

 either of these changes required in even the widest range of vision, is 

 extremely small. For, from the refractive powers of the media of the 

 eye, it has been calculated by Olbers, that the difference between the focal 

 distances of the images of an object at such a distance that the rays are 

 parallel, and of one at the distance of four inches, is only about -143 of 

 an inch. 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 maintain the same form, w r ould not 

 be more than about one line. 



It is now almost universally believed that 

 Helmholtz is right in his statement that the 

 immediate cause of the adaptation of the eye 

 for objects at different distances is a varying 

 shape of the lens, its front surface becoming 

 more or less convex, according to the distance 

 of the object looked at. The nearer the obiect, 



J FIG. 375. Diagram showing 



the more convex does the front surface of the three reflections of a candle, i, 



. From the anterior surface of 



lens become, and Vice Versa: the back Surface cornea; 2, from the anterior sur- 



, , . ..... . face of lens: 3, from the posterior 



taking little 01' no Share in the production Of surface of lens. For further ex- 



~, . -i rm planation, see text. The experi- 



tlie ettect required. The following Simple ex- ment is best performed by em- 



, .,, J ., . . . ploying an instrument invented 



penment illustrates this point. If a small flame by Heimhoitz, termed a Phako- 

 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 (Fig. 375). The first and brightest is (1) 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 and reversed, it is formed at the posterior surface of the lens, 

 which is concave forward, and therefore, like all concave mirrors, 

 gives a reversed image. If now the eye under observation be made 

 to look at a near object, the second image becomes smaller, clearer, and 



