REFRACTION IN THE EYE 495 



them parallel, and its focal power is the measure 

 of the myopia. 



Measurement of Hypermetropia. If the image 

 of the fundus be blurred by hypermetropia, place 

 convex lenses before the eye until the strongest 

 convex lens is found through which the observer 

 can see clearly the retinal vessel or other point 

 selected. The rays emerging from the hyperrne- 

 tropic eye are divergent. This lens renders 

 them parallel, and its focal power is the measure 

 of the hypermetropia. 



Measurement of Astigmatism. Set the retinal 

 tube of the artificial eye at zero. The eye is now 

 emmetropic. Place before the eye the cylindrical 

 lens of 4- 2 D. Examine the fundus with the 

 ophthalmoscope. The observer's accommodation 

 must be relaxed. 



The optic disk will no longer appear circular, 

 but will be elongated in the direction of the 

 meridian of greatest curvature. The retinal ves- 

 sels will not all be in focus. If a horizontal ves- 

 sel be seen distinctly, and the vertical vessel at 

 right angles to it is blurred, the eye is astigmatic 

 in the horizontal meridian (compare page 423, and 

 remember that the breadth of the image of the 

 vessel is determined by means of the rays passing- 

 through that meridian of the cornea which lies at 

 right angles to the vessel's course.) With the aid 



