742 



THE SENSES 



Retinoscopy. Everyone is familiar with the fact that it is quite impos- 

 sible to see the fundus or back of another person's eye by simply looking 

 into it. The interior of the eye forms a perfectly black background to the 

 pupil. The same remark applies to an ordinary photographic camera, 



A 



a, b 



FIG. 468. Diagram to Illustrate the Action of the Ophthalmoscope when a Plane 

 Concave Glass is Used, c, Observer's eye. The light reflected from any point, d, on 

 retina of a, would naturally be focused at e; if the lens b is used is would be focused at **, 

 in other words, at back of c. The image would be enlarged, as though of g, and would be 

 inverted. (After McGregor Robertson.) 



and may be illustrated by the difficulty we experience in seeing into a room 

 from the street through the window unless the room be lighted from 

 within. In the case of the eye this fact is partly due to the feebleness of 

 the light reflected from the retina, most of it being absorbed by the retinal 

 pigment. But the difficulty is due more to the fact that every such ray is 

 reflected back to the source of light and cannot be seen by the unaided eye 

 without intercepting the incident light as well as the reflected rays from 

 the retina. The difficulty is surmounted by the use of the ophthalmoscope. 



The ophthalmoscope, brought into use by Helmholtz, consists in its sim- 

 plest form of a concave mirror with a hole in it. The one described is one 

 of the less intricate of the modern instruments. It consists of, a, a slightly 

 concave mirror of metal or silvered glass perforated in the center, and fixed 



a 



FIG. 469. Diagram to Illustrate Action of Ophthalmoscope when a Biconvex Glass is 

 Used. The figure d on retina of a is under ordinary conditions focused at / and inverted. 

 If the lens b be placed between eyes, the image h is seen by the eye c as an enlarged image. 

 (After McGregor Robertson.) 



into a handle ; and b, a biconvex lens of 6 to 8 cm. focal length. Two methods 

 of examining the eye with this instrument are in common use the direct 

 and the indirect: both methods of investigation should be employed. A 

 normal eye should be examined. A drop of a solution of atropine (two grains 

 to the ounce) or of homatropine hydrobromate should be dropped into the right 

 eye only about twenty minutes before the examination is commenced; the 

 ciliary muscle is thereby paralyzed, the power of accommodation is abolished, 



