9 2o A MANUAL GF PHYSIOLOGY 



and measured. The observer must always keep his eye un- 

 accommodated, and if it is not emmetropic, he must know the 

 amount of his short- or long-sightedness i.e., the strength and 

 sign of the lens needed to correct his defect of refraction, and 

 must allow for this in calculating the defect of his patient. 

 Non-accommodation of the eye of the latter can always be 

 secured by the use of atropine. 



By the direct method of ophthalmoscopic examination, only 

 a small portion of the retina can be seen at a time, and this is 

 highly magnified. A larger, though less magnified, view can 

 be got by the indirect method. The observed eye is illuminated 

 as before, but the mirror and the observer's eye are at a greater 

 distance (Fig. 400). Here the rays from a considerable portion 

 of the retina arc brought to a focus by a convex lens held near 



Fro. 399. 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 eye of the patient, so as to form a real and inverted aerial 

 image of the 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 the direct method to cause dilatation of 

 the pupil by atropine, which also relaxes the accommodation 

 (Practical Exercises, p. 994). 



Skiascopy. To a great extent the ophthalmoscopic method 

 of measuring errors of refraction has been replaced by the more 

 modern method of skiascopy (shadow test). It depends upon 

 the following observation : When one throws light from a little 

 distance with a concave mirror into an observed eye and then 

 rotates the mirror slowly around the long axis of the handle, 

 one sees that the pupil, which at first was completely illuminated, 



