THE SENSE OF VISION. 319 



stereoscopic appearance, and the correct relations between the different parts of 

 the model are at once obvious. 



This imperfection of the eye may be detected by looking at lines such as are 

 shown in Figure 140, and testing each eye separately. If the straight lines 

 drawn in various directions through a common point cannot be seen with equal 

 distinctness at the same time, it is evident that the eye is better adapted to focus 

 rays in one meridian than in another — i. e. it is astigmatic. The concentric 



Fig. 140.— Lines for the detection of astigmatism. 



circles are a still more delicate test. Few persons can look at this figure attentively 

 without noticing that the lines are not everywhere equally distinct, but that in 

 certain sectors the circles present a blurred appearance. Not infrequently it 

 will be found that the blurred sectors do not occupy a constant position, but 

 oscillate rapidly from one part of the series of circles to another. This phe- 

 nomenon seems to be due to slight involuntary contractions of the ciliary 

 muscle causing changes in accommodation. 



The direction of the meridians of greatest and least curvature of the cornea 

 of a regularly astigmatic eye, and the difference in the amount of this curvature, 

 can be very accurately measured by means of the ophthalmometer (see p. 304). 

 These points being determined, the defect of the eye can be perfectly corrected 

 by cylindrical glasses adapted to compensate for the excessive or deficient 

 refraction of the eye in certain meridians. 



By another method known as " skiascopy," which consists in studying the 

 light reflected from the fundus of the eye when the ophthalmoscopic minor is 

 moved in various directions, the amount and direction of the astigmatism of 

 the eye as a whole (and not that of the cornea alone) may be ascertained. 



Astigmatism is said to be irregular when in certain meridians the curvatures 

 of the refracting surfaces are not arcs of circles or ellipses, or when there is a 

 lack of homogeneousness in the refracting media. This imperfection exists to 

 a greater or less extent in all eves, and, unlike regular astigmatism, is incapable 

 of correction. It manifests itself by causing the outlines of* all brilliant objects 

 to appear irregular. It is on this account that the fixed stars do not appear to 

 us like points of light, but as luminous bodies with irregular " star "-shaped 

 outlines. The phenomenon can be conveniently studied by looking at a pin- 

 hole in a large black card held at a convenient distance between the eve and a 

 strong light. The hole will appear to have an irregular outline, and to some 

 eyes will appear double or treble. 



