994 



THE SENSES 



but the mirror and the observer's eye are at a greater distance 

 (Fig. 416). Here the rays from a considerable portion of the 

 retina are brought to a focus by a convex lens held near the eye 

 of the patient, so as to form a real and inverted aerial image of the 



Fig 414. Use of the Ophthalmoscope (Direct Method) for testing Errors of Re- 

 fraction in Myopic Eye. Rays issuing from a point of the retina of E', the 

 observed (myopic and unaccommodated) eye, pass out, not parallel, but con- 

 vergent. They will therefore be focussed in front of the retina of the observing 

 (unaccommodated) eye E if the latter is emmetropic. By introducing a concave 

 lens L of suitable strength, however, a clear view of the retina of E' will be 

 obtained, and the strength of this lens is the measure of the amount of myopia. 



retina. This image is viewed by the observer at his ordinary visual 

 distance. It is not necessary in this method that the observed 

 eye should be non-accommodated, although it is convenient as in 



Fig. 415. 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. 



the direct method to cause dilatation of the pupil by atropine, which 

 also relaxes the accommodation (Practical Exercises, p. 1065). 



Skiascopy To a great extent the ophthalmoscopic method of 

 measuring errors of refraction has been replaced by the more modern 



