THE SENSE ORGANS 197 



point of reversal because it emits divergent rays. Now interpose a 

 convex lens in front of the artificial eye, thereby converging these rays 

 and producing an artificial point of reversal and, so to speak, an artificial 

 myopia. This lens accomplishes two things, namely, it neutralizes the 

 divergence of the rays, and secondly, converges them sufficiently to inter- 

 sect. In order to obtain the degree of hypermetropia existing in this 

 eye, determine the point of reversal as in normal myopia. Subtract 

 the degree of myopia from the total strength of the lens. The remainder 

 of the focal strength of the lens is the strength which is required to over- 

 come the hypermetropia. Draw a diagram showing the reflection in the 

 hypermetropic eye. 



Select a number of students who are either near-sighted or far- 

 sighted and determine the degree of ametropia in each. For example, 

 if the erect movement is obtained as close as 55 cm. from the eye and 

 the reversal as near as 80 cm., the point of reversal lies at a distance 

 of about 67 cm. The myopia equals 1.50 D. Again, supposing that 

 the movement of the light in the pupil is found to be with the mirror at 

 all distances, then interpose a 5 D. (convex) lens. If the point of re- 

 versal is now at a distance of 1 m., 4 D. have been used to neutralize 

 the divergency and 1 D. to render the rays convergent. Consequently, 

 the degree of hypermetropia equals 4 D. 



