THE EYE A3 OPTICAL INSTRUMENT. 517 



camera provision exists for adjusting it to the varying 

 distances of objects : this consists in sliding the lens in 

 and out, and thus changing its distance from the 

 ground -glass plate according as the images of near or 

 of distant objects are to be received on the plate. A 

 similar capability of adjustment of the eye enables a 

 healthy eye to accommodate itself to the varying dis- 

 tances of objects and to form distinct images of them 

 upon the retina. Various changes in the eye contribute 

 to this adjustment. The crystalline lens is not only moved 

 forwards and backwards like the lens of a camera, but 

 its convexity is also increased or diminished by the 

 varying pressure of muscles which act upon the edges 

 of the lens, and thereby alter its focal length. If 

 the focal length is thus somewhat increased, the image 

 of a distant object is formed at the same distance from 

 the lens as the image of a nearer object produced by a 

 lens of shorter focal length. Adjustment can take place 

 only within a certain range, which admits of great indi- 

 vidual variations. As a rule, no object which is brought 

 within less than 20 cm from the eye can be seen distinctly 

 without effort. But in many persons the power of 

 adjustment is limited in consequence of disease or want 

 of practice. Many persons are born with the surface of 

 the cornea more convex than usual, others, (for example, 

 engravers, designers), whose employment necessitates 

 a preponderant application of the eye to near objects, 

 lose the power of adjustment to distant ones ; these 

 are short- sighted. Others again for example, seamen 

 who have mostly to look at distant objects, gradu- 

 ally lose the power of adjustment for near objects. 

 They become long-sighted. Elderly people are also 

 usually long-sighted, for as age draws on the cornea 



