CH. LVITI.] THE OPHTHALMOSCOPE 837 



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

 background.* The same remark applies to the difficulty we experi- 

 ence 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 examina- 

 tion 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 Helm- 

 holtz's time ; the principal modification is the substitution of a con- 

 cave 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 hoffiatropine hydrobromate, should be 

 instilled about twenty minutes before the examination is commenced ; the ciliary 

 muscle is thereby paralysed, the power of accommodation is abolished, and 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 somewhat 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 of the patient. A red glare, known as the reflex, is 

 seen ; it is due to the illumination of the retina. The patient is then told to look 

 at the little finger of the observer's right hand as he holds the mirror; to effect 

 this the eye is rotated somewhat inwards, and at the same time the reflex changes 

 from red to a lighter colour, owing to the reflection from the optic disc. The 



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

 retinal epithelium ; this forms the Tapatum lucldum. 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 probably the case that these animals are able to see 

 clearly with less light than we can, hence the popular idea 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. 



