368 PEACTICAL PHYSIOLOGY 



being to the left and the rectanglar area to the right. A white line on 

 the back of the right illuminated area will now point to some number 

 on the scale ; when the images are adjusted as above, this number + the 

 20 corresponding to the position of the left illuminated area, will express 

 numerically the degree of curvature of the cornea. According to the 

 constants of the instrument if the number 337 be divided by the number 

 expressing the curvature of the cornea as above, the quotient represents 

 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. 241 the difference of curvature 

 of different portions of the cornea can be easily ascertained. 



FIG. 242. The images in the astigmometer. 



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 refract- 

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

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

 most refracting meridian may thus be read off at once. 



1 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. 



