124 ELEMENTARY PHYSIOLOGY. XXIII. 



17. Negative after-image. Look fixedly for about 

 twenty seconds 



a. At a white patch (e.g. white wafer) on a black 

 ground, and then look at a white surface (or preferably 

 pass a white surface over the whole, keeping the visual 

 axis fixed); there will be visible a corresponding dark 

 patch on the white ground. 



6. At a black patch on a white ground, and turn to a 

 grey surface; there will be visible a white patch on a 

 grey ground. 



c. At a red patch on a black ground, and turn to a 

 white surface, there will be visible a blue-green patch. 



And so with the other colours, the colour of the nega- 

 tive image will be complementary to that of the actual 

 object. 



d. At a red patch on a black ground, and turn to a 

 yellow surface; there will be visible a green patch. 



e. Look fixedly at a brightly illuminated window and 

 then close the eye. The positive after-image will probably 

 not be seen; in its place there will come the negative 

 after-image with the sashes as bright lines and the panes 

 as dark spaces. This will in turn be succeeded by coloured 

 images. 



18. Contrast, a. Cut out two strips of the same 

 grey paper. Place one on white paper and the other on 

 black paper (or better still, velvet). The strip on the 

 black surface will appear distinctly brighter than the 

 other. Arrange that the two strips are close to one 

 another, look fixedly at them for 20 seconds and then 

 close the eyes. The after-image of the strip on the black 



