VISION 123 



seen on a bright ground, B immediately announces, 

 "negative after-image." A now cuts off the light of the 

 candle from the screen by means of an opaque object, 

 and B will see the negative image change to a bright 

 image on a dark ground the positive after-image. 



A uncovers the candle and the image becomes nega- 

 tive again. This process may be repeated for some time, 

 6 minutes or longer, but it will be found that there are 

 periods during which both images temporarily disap- 

 pear. Important points in the successful working of the 

 experiment are that B should sit quite still and should 

 keep his eyes fixed on the centre of the tracing paper 

 screen during the observation of the images. 



As a variation of the above experiment the candle may be moved 

 to and fro. so as to vary the intensity of illumination of the tracing 

 paper screen. 



16. Positive after-image. When waking in the 

 morning, close and shade the eyes for a minute or two, then 

 suddenly look at the bright window for a moment or two, 

 and then close and shade the eyes again. The image of 

 the window exactly corresponding to the natural one, 

 i.e. with the sashes dark and the panes bright, &c., will 

 last for some little time, i.e. the sensation is of longer 

 duration than the application of the stimulus. 



To succeed, the retina should be in rest beforehand, 

 and the exposure to the stimulus momentary or 

 nearly so. 



Or, in the evening, having closed and shaded the eyes 

 for some time, suddenly look at a lamp and immediately 

 close the eyes. A similar positive after-image will be 

 seen. 



