OPHTHALMOSCOP Y 



297 



up the interior it would become visible by refieetecl light, just 

 as we can see into a lighted room at night if the window is not 

 covered with a blind. When we try to illuminate the interior of 

 the eye, we find that we nuist interpose our head between it and 

 the source of light in our attempts to peer into it. This difliculty 

 is overcome by reflecting light from a mirror provided with a small 

 central aperture, through which the observer can look. This 

 instrument is called an ophthalmoscope. It may be used in two 

 ways : 



A. In direct ophthalmoscopy the mirror and the observer's eye 

 are brought close to the observed eye, and the refracting media 



AERIAL 



IMAGE 



OF FATIENT'S 



R.ETINA 



Fig. 81. — Diagram to show paths of rays from eye of patient (on left) to observer when tlie 

 indirect method of ophthalmoscopy is in use (Hartridge). 



of the latter produce a virtual image, erect and magnified, of the 

 retina. The lens, etc., of the observed eye act in exactly the same 

 way as a magnifying glass, the object being just inside the focus. • 

 B. In indirect ophthalmoscopy the observer holds a convex 

 lens in front of the observed eye, and places himself farther 

 away. The interposed lens brings the rays leaving the observed 

 eye to a focus between itself and the observer, who consequently 

 sees an inverted image of the retina. This is real and magnified, 

 the magnification depending on the lens used (Fig. 81). 



Further Reading 



Edridge-Green. " The Physiology of Vision." Bell. 

 GouLDEN. "Refraction of the Eye." J. & A. Churchill. 



