io66 THE SENSES 



solution of atropine sulphate, but this is not indispensable for the 

 experiment. 



(a) Direct Method. A takes the mirror in his right hand, and, 

 holding it close to his own eye, looks through the central hole, and 

 throws a beam of light into B's eye. A red glare, the so-called ' ixflex ' 

 from the choroidal vessels, is now seen. A then brings the mirror 

 to within 2 or 3 inches of B's eye, keeping his own eye always at the 

 aperture. A and B both relax their accommodation, as if they were 

 looking away to a distance. If both eyes are emmetropic, the retinal 

 vessels will be seen. B should now look away past the little finger of 

 A's right hand. This causes slight inward rotation of B's eye, and 

 brings into view the white optic disc with the central artery and vein 

 of the retina crossing it. 



(b) Indirect Method. A takes the mirror in his right hand to ex- 

 amine B's right eye, places his own eye behind the aperture as before 

 at a distance of about 18 inches from B, and throws a beam of light 

 into B's eye. Then A takes a small biconvex lens in his left hand, and 

 places it 2 or 3 inches in front of B's eye, keeping it steady by resting 

 his little finger on B's temple. A now moves the mirror until he sees 

 the optic disc. 



(2) Examine a rabbit's eye by the direct and indirect method. 

 Dilate the pupil by a drop or two of atropine solution. 



For practice, before doing (i) and (2) the student should examine 

 an artificial ' eye ' by both methods, so as to get a clear view of what 

 represents the retina. A substitute for the artificial eye may be made 

 by unscrewing the lower lens of the eyepiece of a microscope, and 

 fastening in its place a piece of paper with some printed matter on it. 

 The letters must be made out with the ophthalmoscope. 



The opportunity should also be taken to observe the eye of an 

 anaesthetized animal by the simple cover-glass method mentioned 

 in i (p. 1058). Around cover-glass is slipped under both eyelids and so 

 held in position on the .cornea. The fundus of the eye can now be 

 clearly seen, including the optic disc and retinal vessels. The instilla- 

 tion of a little cocaine into the eye of a rabbit will produce local an- 

 aesthesia sufficient to permit the experiment. 



13. Skiascopy or Retinoscopy. The simplest method is as follows: 

 The observer places himself at a distance of a metre from the observed 

 eye, .which he illuminates by a beam reflected from a concave ophthal- 

 moscopic mirror held in front of his eye. The accommodation of the 

 observed eye is relaxed. If, now, when the mirror is rotated no direc- 

 tion of movement of the shadow or the light area (p. 995) can be made 

 out, the pupil becoming all at once dark throughout its whole extent 

 when the mirror is rotated in one direction, and all at once light 

 throughout its whole extent when the mirror is rotated in the opposite 

 direction, the observer is in the far-point of the observed eye. Since 

 the far-point is at the distance of a metre, there is in this case myopia 

 amounting to one diopter. If, however, the light area moves in the 

 same direction as the rotation of the concave mirror, the far-point of 

 the observed eye lies between the observer and the observed eye, so 

 that the myopia amounts to more than one diopter. The precise 

 degree of myopia can be estimated by interposing biconcave lenses of 

 different strength until the far-point is made just i metre. 



If the light area moves in the opposite direction to the rotation 

 of th? mirror, the far-point is more than a metre distant, and therefore 

 the observed eye is emmetropic or hypermetropic, or myopic to a degree 

 less than a diopter. The lens, convex or concave, can now be sought 

 out which will just bring the far-point to a metre, and from the strength 



