232 PRACTICAL PHYSIOLOGY 



the radius of curvature of the cornea in the horizontal meridian 

 examined. The use of the instrument for measuring astigmatism may 

 here be detailed. 



EXPERIMENT. Method of Measuring Astigmatism. By the use of 

 the ophthalmometer represented in Fig. 211 the difference of curvature 

 of different portions of the cornea can be easily ascertained. 



FIG. 211. The images in the astigtnometer. 



The apparatus is adjusted as described above, and the horizontal 

 meridian is first observed. If the curvature in this meridian is regular 

 the four figures will be seen to stand on a level base. If this is not the 

 case, the rotating quadrant must be moved till continuity of base line is 

 obtained. The moveable illuminated area is then adjusted till the four 

 reflected images are as in the figure. 



The quadrant is then rotated, and as it approaches the vertical the 

 two central images will probably overlap. Note the meridian where the 

 greatest amount of overlap is observed. This will be the most retract- 

 ing meridian. Each tread of the steps in the illuminated area corre- 

 sponds to one dioptre 1 of curvature. The excess of curvature of the 

 most refracting meridian may thus be read off at once. 



CHAPTER LXVII. 



THE REFRACTING MEDIA OF THE EYE. 



Ktihne's Artificial Eye. The nature of the refraction produced by 

 the various media of the eye is conveniently illustrated by means of 

 this instrument (Fig 212). It consists of an oblong box, one of the 

 long vertical sides being generally made of opaque material, the other 

 of glass. The front end of the box is bounded by a curved glass 

 surface, the hinder end is a plane sheet of glass. Various accessories 

 are supplied with the instrument, such as a double convex lens which 

 can be placed in the axis of the box behind the cornea, a frosted glass 



J A lens in which the focus for parallel rays is at one metre is taken as the 

 standard lens, and its degree of refractive power is represented as one dioptre. 



