802 



A MANUAL OF PHYSIOLOGY 



ing from the unaccommodated eye are divergent, and a 

 convex lens, the strength of which is proportional to the 

 amount of hypermetropia, must be placed before the obser- 

 ver's unaccommodated eye if he is to see the fundus dis- 



FIG, 303. USE OF THE OPHTHALMOSCOPE (DIRECT METHOD) FOR TESTING 

 ERRORS OF REFRACTION IN MYOPIC EYE. 



Rays issuing from a point of the retina of E' ( the observed (myopic and unaccom- 

 modated) eye, pass out, not parallel, but convergent. They will therefore be focussed 

 in front of the retina of the observing (unaccommodated) eye E if the latter is einme- 

 tropic. By introducing a concave lens L of suitable strength, however, a cle^r view 

 of the retina of E' will be obtained, and the strength of this lens is the measure of the 

 amount of myopia. 



FIG. 304. TESTING ERRORS OF REFRACTION IN HYPERMETROPIC EYE. 



Rays from a point of the retina of E', the observed eye, issue divergent, and are 

 focussed behind the retina of the observing (unaccommodated and emmetropic) eye E. 

 The strength of the convex lens L, which must be introduced in front of E to give clear 

 vision of the retina of E', measures the degree of hypermetropia. 



tinctly. By accommodating, the observer can see the fundus 

 clearly without a convex lens. 



By this method errors of refraction in the eye may be 

 detected and measured.* The observer must always keep 



* To a great extent the opthalmoscooic method of measuring errors of 

 refraction has been replaced by the more modern method of skiascopy 

 (shadow test), which, however, it would be out of place to describe here. 



