PRACTICAL EXERCISES 86 1 



zontal limbs of a cross cut out of a piece of cardboard and placed in 

 the path of the beam of light cannot be both focussed at the same 

 time. 



5. Mapping the Blind-spot. Make a black cross on a piece of 

 white paper attached to the wall, the centre of the cross being at the 

 height of the eye in the erect position. Stand about 12 inches from 

 the wall, the chin supported on a projecting piece of wood. Fix the 

 centre of the cross with one eye, the other being closed, and move 

 over the paper a pencil covered, except at the point, with white paper, 

 until the point just disappears. Make a mark on the paper at this 

 point, and repeat the observation for all diameters of the field. The 

 blind-spot is thus marked out (Fig. 332). Its shape is not the same 

 in all eyes (Fig. 333). Its size and distance from the fovea centralis 

 can be calculated from the construction given in Fig. 291. 



6. Ophthalmoscope (i) Human Eye (p. 800). 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 

 5 per cent, solution of atropia 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 atropia 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. 



7. Pupillo- dilator and Constrictor Fibres. (a) Set up an induc- 

 tion machine arranged for tetanus, and connect a pair of electrodes 



