DIOPTRICS OF THE EYE. 



331 



cornea are each doubled, when viewed through the telescope, by means of 

 a double vision prism of Iceland spar and the displacement produced in 

 this way is a definite amount for the distance chosen. Four images of the 

 mires are thus seen, and when the mires are properly adjusted for a cornea of 

 average curvature the two inner images are in contact with each other. 

 A variation from this average is indicated by an overlapping of the images. 



Fig. 140. — Ophthalmometer (Hardy). 



the value of which in diopters or in radii of curvature is read off upon the 

 instrument. The instrument, therefore, when once calibrated enables one 

 to read off at once the radii of curvature for the different meridians and thus 

 determine the axis and degree of astigmatism. It should be added that 

 the instrument gives only the curvatures and degree of astigmatism, if any 

 exists, of the cornea, and is therefore of no immediate service in determining 

 the total astigmatism, that is, the astigmatism of cornea and lens acting 

 together. 



