496 THE OUTGO OF ENERGY 



of the graduated circle on the front of the artifi- 

 cial eye determine the meridian in which the eye 

 is astigmatic. Find the lens which will make the 

 blurred vessel distinct. If the lens, for example, 

 have a focal power of + 2 D, there is simple hyper- 

 metropic astigmatism of + 2 D in the given 

 meridian. If a lens of 2 D be required, there 

 is simple myopic astigmatism of 2 D in the 

 given meridian. 



In compound astigmatism, the eye is asymmet- 

 rical in more than one meridian. Thus a clear 

 image of the vertical vessels may be obtained 

 with a convex lens of + 2 D, while the horizontal 

 vessels may require a lens of -f 1 D. 



The ophthalmoscopic measurement of astigma- 

 tism in the human eye is exceedingly difficult, 

 and should always be corrected by more reliable 

 methods. 



INDIRECT METHOD 



1. Arrange the light and the artificial eye as 

 directed for the examination by the direct 

 method. Hold the ophthalmoscope 30 cm. from 

 the artificial eye. With the other hand hold a 

 convex lens of 20 D at its own focal length of 

 5.0 mrn. in front of the cornea. The rays return- 

 ing from the fund us pass through this lens and 

 form an image in the air between the observer 



