360 PRACTICAL PHYSIOLOGY. [LXXIV. 



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

 image corresponding to that looked at will be seen. 



{b.) Rest the retina by closing the eyes, then look at a gas- 

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

 wliich 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.) Rest 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 black 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 Uning optical apparatus. Notice also while staring at the white 

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

 to contrast. 



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

 the eyes. 



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

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



{(i.) 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.j 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 

 w^ith a sheet of white paper : a greenish-blue after-image wiU 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 



