994 A MANUAL OF PHYSIOLOGY 



(2) Keep the eye closed for a short time. Then direct it to a 

 surface illuminated by a weak blue light. A dark blue or almost 

 black spot (Maxwell's spot), corresponding to the macula, is seen in 

 the visual field, owing to the absorption of the blue rays. 



12. Ophthalmoscope (i) Human Eye (p. 918). Let A be the 

 observer, and B the person whose eye is to be examined. A and B 

 are seated facing each other. Suppose that the right eye of B is to 

 be examined. Close to the left ear of B is a lamp on a level with 

 his eyes : the room is otherwise dark. For a clinical examination, 

 the pupil should be dilated by putting into the eye a drop of a 

 0-5 per cent, 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 

 ' reflex ' 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 

 examine 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. 985). A round 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 

 anaesthesia 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 ophthalmoscopic mirror held in front of his eye. The 

 accommodation of the observed eye is relaxed. If, now, when the 

 mirror is rotated no direction of movement of the shadow or the light 

 area (p. 920) can be made out, the pupil becoming all at once dark 

 throughout its whole extent when the mirror is rotated in one direc- 

 tion, 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 



