286 ELEMENTARY PHYSIOLOGY. [XXVII. 



and immediately close the eyes. A similar posi- 

 tive after-image will be seen. 

 This positive after-image must not be confounded 

 with the negative after-image which comes later. 

 It simply shews that the sensation is of longer 

 duration than the application of the stimulus. 



16. Look for an instant at a coloured light, and 

 then look at a white or grey surface, a positive 

 image of the same colour will for a brief period 

 be visible. 



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. 



5. 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 negative image will be complementary to 

 that of the actual object. 



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



