300 DETAILED SYNTHESIS. 



manner, that the light from objects within reasonable 

 distance shall reach a focus at the retina. 



795. Remark. THE CURVATURE OF THE RETINA certainly, and per- 

 haps also its distance from the lens, is affected by the external muscles 

 of the eye, yet to be described. 



796. THE EYE CHANGES, WHEN PERFECT, from that 

 of short to that of long sight without effort, but in long- 

 sighted persons it cannot be changed to short-sight, and 

 in the case of the short-sighted it cannot be changed to 

 long sight. 



797. Remark. THE WORDS SHORT-SIGHTED AND LONG-SIGHTED do 

 not convey a correct idea, for" neither is anything more than a perfect 

 eye, but fails in being something less. The short-sighted is no more so 

 than any perfect eye, but is not long-sighted. 



798. Remark. It is, however, seldom that LONG or SHORT-SIGHTED 

 PERSONS ARE RESTRICTED to a single distance, their eyes nearly always 

 having some power of adaptation, but not sufficient for perfect sight. 



799. Remark. In eyes called good there is A GREAT DIFFERENCE 

 IN THE DEGREE OF ADAPTATION : one person can see at a few inches' or at 

 miles' distance, while another has a more restricted range. All eyes have 

 their limit, and those only in case of which it is inconvenient are noticed. 



FIG. 198. 



800. IN THE CASE OF 



I THE LONG-SIGHTED at ordi- 

 nary distances for reading, 

 | the light from A, Fig. 1 98,' 

 is not brought to a focus at 

 a, as it would be in a per- 

 fect eye, but spreads over 

 the surface from b to c, on account of which A will seem 

 to occupy all the space from B to C, since, from the ne- 

 cessary constitution and action of the eye, the light act- 

 ing on b will appear to come from B, whether it does or 

 not, and the light acting at c will appear to come from 

 (7, and the light acting on all the visual points between 

 b and c will appear to come from an equally numerous 

 series of objects between B and C. 



795. What sai.l of -? 796. How does -? 797. What said of ? 799. How 

 arc ? 799. Is there ? 800. What the condition 



