996 A MANUAL OF PHYSIOLOGY 



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

 of the mirror, the far-point is more than a metre distant, and there- 

 fore 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 of it, minus one diopter, the refraction can 

 be estimated. Suppose, for instance, that a convex lens of two 

 diopters is required, then hypermetropia of one diopter exists. 



In order to facilitate the introduction of the various lenses, 

 instruments called skiascopes or retinoscopes may be used, one of 

 which is shown in Fig. 443. 



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

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

 through a short-circuiting key with the secondary. Etherize a cat 

 by putting it into a large vessel with a lid, slipping into the vessel 

 a piece of cotton-wool soaked with ether, and waiting till the move- 

 ments of the animal inside the vessel have ceased. Then quickly 

 put the cat on a holder and maintain anaesthesia with ether. Expose 

 the vago-sympathetic in the neck (pp. 148, 202) ; the carotid is taken 

 as the guide to it. Ligature the nerve and cut below the ligature. 

 On stimulating the upper (cephalic) end, the pupil of the corre- 

 sponding eye dilates. 



Carefully separate the sympathetic from the vagus, and repeat the 

 observation on the former. The result on the pupil is the same. 



(b) Observe in the eye of a fellow-student, or, by means of a 

 looking-glass, in your own eye, that when light falls on one eye both 

 pupils contract. 



(c) Observe that when the eye is accommodated for a near object 

 the pupil contracts, and that it dilates when a distant object is 

 looked at. 



15. Colour-mixing. (a) Arrange a red and a bluish-green disc on 

 one of the steel discs of the colour-mixing apparatus shown in 

 Fig. 444, so that a part of each is seen. On another arrange a violet 

 and a yellow disc, and on the third an orange and a blue disc. By 

 adjustment of the proportions of the two colours a uniform grey 

 can be obtained from each of these combinations (complementary 

 colours) when the discs are rapidly rotated. 



(b) Mix two colours that are not complementary e.g., blue and 

 red grey or white cannot be obtained by any adjustment of pro- 

 portions ; the result is always a mixed colour, the precise hue 

 depending on the amount of each ingredient. 



(c) Take papers of any three colours from widely-separated parts 

 of the spectrum e.g., blue, green, and red and arrange them on one 

 of the rotating discs. By varying the proportions, white (grey) can 

 be produced, and any other coloured paper fastened on another of the 

 rotating discs can be matched by adding white to the three colours. 



1 6. After-images (i) Positive. (a) Rest the eyes for two or three 

 minutes by closing them, or by going into a dark room. Then 

 look for an instant at a bright object, a window or an incandescent 

 lamp, and at once close the eyes again. A bright positive after- 

 image of the object looked at will be seen. 



(b) Look at an incandescent lamp through a coloured glass as in (a) . 

 The positive after-image will appear in the same colour as the glass. 



(2) Negative After-image. (a] Look at an incandescent lamp for 

 thirty seconds, and then direct the eyes to a white surface. The 

 after-image of the filament will appear dark. 



