NEGATIVE AFTERIMAGES. 293 



9. Binocular Contrast. 



(a.) Place a white strip of paper on a black surface, look 

 at the white paper and squint so as to get a double image. 

 In front of the right eye place a blue glass, and in front 

 of the left one a grey (smoked) glass. The image of 

 the right eye will be blue, that of the left yellow. Instead 

 of the grey glass, a card with a small hole in it placed in 

 front of the left eye does perfectly well. The yellow of the 

 left eye is a contrast sensation. 



10. Positive After-images. 



(&.) In a room not too brightly illuminated, rest the retina 

 by closing the eyes for a minute or two, then suddenly look 

 for a second or two at a gas jet surrounded with a white 

 globe, then close the eyes. An image corresponding exactly 

 to that looked at will be seen. 



(6.) After resting the retina by closing the eyes, 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 condi- 

 tion within the eye. 



11. Negative After-images. 



(a.) Rest the retina, and then stare steadily for half a 

 minute or less at a small white square or white cross on a 

 dead black ground. To insure fixation of the eyeballs, make 

 a small mark in the centre of the white paper, and fix this 

 steadily. In all subsequent experiments do the same. 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 dead 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. 



