270 



The Evolution of Optics. 



near-sight. 



eye, according to the degree of myopia, is normally adapted 

 for objects more or less near. 



Still another defect more annoying than either of the 

 foregoing is caused by the fact that the curvature of the 

 anterior surface of the eye is not 

 symmetrical that is, should we 

 make a section of the eye in the 

 horizontal and again in the verti- 

 cal meridian, we would find that 

 one segment was longer than the 

 other, because the cornea is more 

 sharply curved in one direction 

 than in the other. As a necessary 

 result, the light entering the eye 

 in these different meridians can 

 not unite in one f ocus that is, it 

 could no longer be a point, but a 

 line of light and from this fact 

 the defect is called astigmatism, 

 the name signifying without a point. The annoying visual 

 disturbance which this defect causes is very frequently over- 

 come by an unequal contraction of the focusing muscle, 

 making the lens cylindrical to compensate for the corneal 

 defect ; but when not so corrected it gives rise to very curi- 

 ous visual impressions. For example, it is sometimes im- 

 possible for such a person to see the hands of a clock save 

 when they are turned in one direction, being able only to 

 tell the time, say, at twelve and six o'clock. Astigmatism 

 may occur in connection with either far or near sight, or 

 both defects may be combined in the same eye, one meridian 

 being short-sighted, the other long-sighted. 



But such defects of form are by no means the only ones 

 which the eye presents when considered as an optical in- 

 strument. Although the visual field of the human eye is 

 very large, that portion of the sensitized plate capable of 

 appreciating clearly defined images is very small, because 

 one portion of the retina has become more highly special- 

 ized than the remainder; this spot is called the macula 

 lutea, and it is only in the comparatively restricted portion 

 of the field which this macula commands that the most per- 

 fect visual acuity is reached. It is consequently necessary, 

 when carefully examining any object, that the eye should be 

 directed in turn to each point which we wish to observe. 

 This restless motion of the eye gives what we know as ex- 



