CH. LV.] THE OPHTHALMOSCOPE. 7/1 



black background.* The same remark applies to an ordinary 

 photographic camera, and may be illustrated by the difficulty 

 we experience in seeing into a room from the street through 

 the window unless the room is lighted 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 far more to the fact that every such ray is 

 reflected straight to the source of light (e.g., candle), and cannot, 

 therefore, be seen by the unaided eye without intercepting the 

 incident light from the candle, as well as the reflected rays 

 from the retina. This difficulty is surmounted by the use of the 

 ophthalmoscope. 



The ophthalmoscope was invented by Helmholtz ; as a mirror 

 for reflecting the light into the eye, he employed a bundle of 

 thin glass plates ; this mirror was transparent, and so he was 

 able to look through it in the same direction as that of the rays 

 of the light it reflected. It is almost impossible to over-estimate 

 the boon this instrument has been to mankind ; previous to this 

 in the examination of cases of eye disease, the principal evidence 

 on which the surgeon had to rely was that derived from the 

 patient's sensations ; now he can look for himself. 



The instrument, however, has been greatly modified since 

 Helmholtz's time ; the principal modification being the substitu- 

 tion of a concave mirror of silvered glass for the bundle of glass 

 plates ; this is mounted on a handle, and is perforated in the 

 centre by a small hole through which the observer can look. 



The methods of examining the eye with this instrument are the direct 

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

 drop of a solution of atropine (two grains to the ounce) or of homatropine 

 hydrobromate, should be instilled about twenty minutes before the 

 examination is commenced ; the ciliary muscle is thereby paralysed, the 

 power of accommodation is abolished, nnd the pupil is dilated. This will 

 materially facilitate the examination ; but it is quite possible to observe all 

 the details to be presently described without the use of such drugs. The 

 room being now darkened, the observer seats himself in front of the 

 person whose eye he is about to examine, placing himself upon a some- 

 what higher level. Let us suppose that the right eye of the patient is 

 being examined. A brilliant and steady light is placed close to the left 

 ear of the patient. Taking the mirror in his right hand, and looking 

 through the central hole, the operator directs a beam of light into the eye 



* In some animals (t-.g., the cat), the pigment is absent from a portion 

 of the retinal epithelium ; this forms the Tapetum lucidum. The use of 

 this is supposed to be to increase the sensitiveness of the retina, the light 

 being reflected back through the layer of rods and cones. It is certainly 

 the case that these animals are able to see clearly with less light than we 

 can, hence the popular idcn that a cat can see in the dark. In fishes a 

 tapetum lucidum is often present ; here the brightness is increased by 

 crystals of guanine. 



3 D 2 



