I068 THE SENSES 



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 movements 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. 201, 210); 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 corresponding 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 ob]ect 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. 460, 

 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 proportions; 

 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. 



16. 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. 



(b) Look at the lamp through a coloured glass for thirty or forty 

 seconds, and then close the eye or look at a white ground. The after- 

 image of the filament will appear in the complementary colour of the glass. 

 If the glass was red, for instance, the after-image will be greenish. 



(c) Look at a white square on a dark ground for thirty seconds, 

 then quickly cover the field with white paper. A dark square will be 

 seen on the white ground. 



(d) Repeat (c} with coloured squares. The after-image of the 

 square will be in the complementary colour. 



Contrast. Perform Meyer's experiment (p. 1016). 



17. Retinal Fatigue. Fix the eye steadily on a portion of a printed 

 page a considerable distance away. Note that the print soon be- 

 comes blurred. Wink the eye; the short rest causes a notable recovery 

 of the retina. 



1 8. Visual Acuity. Draw on a white card a series of vertical black 

 lines i millimetre thick, and separated from each other by a distance 



