360 PRACTICAL PHYSIOLOGY. 



gas-jet surrounded with a white globe, then close the eyes. An 

 image corresponding to that looked at will he seen. 



(b.) Rest the retina hy closing the eyes, then look at a gas- 

 flame surrounded with a coloured glass, or look at a gas-flame in 

 which some substance is burned to give a characteristic flame, e.g., 

 common salt. Then look at a white surface, when a positive after- 

 image of the same colour will be seen. In all these cases the 

 image moves as the eye is moved, showing that we have to do with 

 a condition within the eye. 



13. Negative After-images. These are regarded as a sign of 

 retino-cerebral fatigue. Successive Contrast. 



(a.) Eest the retina, and then stare steadily for half a minute or 

 less at a small white square or white cross on a black ground. To 

 ensure fixation of the eyeballs, make a small mark in the centre of 

 the white paper, and fix this steadily. Then suddenly slip a sheet 

 of white paper over the whole, a blade square or cross will appear 

 on the white background. I find that the best black surface to use 

 is the dull black of the " Tuch-papier," such as is used by opticians 

 for lining optical apparatus. Notice also while staring at the white 

 paper that its margins appear much brighter than the centre, owing 

 to contrast. 



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

 the eyes. 



(e.) Look at a black square or cross on a white ground. Turn 

 to a grey surface, when a white square or cross will appear. 



(d.) 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 " negative coloured 

 after-image." 



(/.) Place a small red and a small 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. 



These negative after-images are examples of so-called " Succes- 

 sive Contrast." 



14. Haploscope (dTrXoo? single). 



Place the eyeballs in the primary position, i.e., look straight 

 ahead at a hypothetical object on a level with the eyes, but placed 

 at the horizon. The visual axes are parallel, and we have two 

 distinct and separate fields of vision. On looking through two 



