THE SENSES. 60 T 



have been great. The stimulus increases as the ordinary numbers, 

 while the sensation increases as the logarithm. 



The Ophthalmoscope. Part of the light which enters the eye is 

 absorbed, and produces some change in the retina, of which we shall 

 treat further on; the rest is reflected. 



Every one is perfectly familiar with the fact that it is quite impossible 

 to see fhefundus 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, 

 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 

 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 

 choroid, as mentioned above; but far more to the fact that every such 

 ray is reflected straight back 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, brought into use by Helmholtz, consists in 

 its simplest form of a, a slightly concave mirror of metal or silvered 

 glass perforated in the centre, and fixed into a handle; and b, a bicon- 

 vex lens of about 24-3 inches focal length. Two methods of examining 

 the eye with this instrument are in common use the direct and indirect; 

 both methods of investigation should be employed. A normal eye 

 should be examined; a drop of a solution of atropia (two grains to the 

 ounce) or of hom-atropia hydrobromate, should be instilled about twenty 

 minutes before the examination is commenced; the ciliary muscle is 

 thereby paralyzed, 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 with- 

 out the use of this drug. 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. A brilliant and steady 

 light is placed close to the left ear of the patient. The atropia having 

 been put into the right eye only of the patient, this eye is examined. 

 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 color, owing to the reflection from the optic disc. The 

 observer now approximates the mirror, and with it his eye to the eye of 

 the patient, taking care to keep the light fixed upon the pupil, so as not 

 to lose the reflex. At a certain point, which varies with different eyes, 

 but is usually when there is an interval of about two or three inches 

 between the observed and observing eye, the vessels of the retina will be- 

 come visible as lines running in different directions. Distinguish the 



