VISION. 



815 



Ophthalmoscope. This is a small concave mirror by means of 

 which rays of light are directed through the pupil of the eye so that 

 the deep parts are illuminated and made visible. There is a hole 

 in the center of the mirror through which the examiner looks. But 

 the ophthalmoscope may be used with or without lenses. Without 

 lenses the ophthalmoscope gives an erect image. If, however, we 

 use a convex lens over the central aperture of the ophthalmoscopic 

 mirror the observer sees a re-inverted image. If a concave lens is 

 used over the aperture of the ophthalmoscopic mirror there is seen 

 an erect image considerably magnified. The instrument is usually 

 fitted with a series of concave and convex mirrors, which can be 

 revolved in front of the central aperture of the mirror. 



If the observer is myopic he can use the concave lenses to cor- 

 rect his myopia. If he is long-sighted, he corrects it by means of 

 one of the convex lenses. 



Fig. 368. The McHardy Perimeter. ( BROWN.) 



If the eye examined be short- or long-sighted, the retinal image 

 could not be brought into focus with the mirror alone, but the 

 examiner can adjust his concave or convex disc, as the case may be, 

 and find a lens to correct the short or long sight of the eye examined. 



In this way the ophthalmoscope may be used to measure the 

 degree of myopia or hypermetropia of the eye examined. 



Perimeter. It has been noted that by the peripheral parts of 

 the retina a person can observe pretty definitely the form and color 

 of objects. To determine just how far this field of indirect vision 

 extends in every direction from the visual axis is to locate, by the 



