294 EXPERIMENTAL PHYSIOLOGY. 



(b.) The black negative after-image may also be seen by 

 closing the eyes. 



(c.) Look at a dull, dead, black square or cross on a white 

 ground. Turn to a grey surface, when a white square or 

 cross will appear. 



Stare intensely at a bright red square on a black 

 surface for twenty seconds, and then look at a white surface, 

 a bluish-green patch on the white is seen. It waxes and 

 wanes, and finally vanishes. 



(e.) A green stared at in the same way gives a red i.e., in 

 each case the complementary colour is obtained as a "nega- 

 tive coloured after-image." 



(f.) Place a small red and a green square side by side on 

 a black background, stare at them, and quickly cover the 

 whole with a sheet of white paper, a greenish-blue after- 

 image will appear in place of the red, and a reddish-purple 

 instead of the green. 



12. The Haploscope consists of two tubes about an inch and 

 a quarter in diameter and 8 inches in length, which can be con- 

 verged or diverged from a fixed point as desired. On looking 

 through the tubes with both eyes, each eye has its own special 

 field of vision, but with the proper convergence the two fields 

 are united, and form one field of vision. 



(a.) With the discs provided for you copies of those of 

 Volkmann study the combinations obtained in the single 

 field of vision. 



(6.) Struggle of the Fields of Vision. Place in one tube a 

 red and in the other a green glass. One sees either a red or 

 green disc, but not a mixture of the two colours. 



13. Stereoscope. 



(a.) Examine a series of stereoscopic slides to show the 

 combination of the images obtained by the right and left 

 eyes respectively a slide to show lustre, and another of 

 different colours on the two sides. 



