288 EXPERIMENTAL PHYSIOLOGY. 



the eyes there is no partial darkening of the white ground, 

 but only an intensely bright image of the strip. 



5. Contrast. 



(a.) On the rotating machine cause a disc, as in Fig. 138, 

 to rotate with moderate rapidity, when several zones will be 

 seen, the innermost black, while each one further outwards 



Fig. 138. Disc for Contrast. 



is lighter in tint. Each zone, where it abuts against the 

 inner darker zone, is lighter than the rest of the same zone, 

 and shades oft" gradually to the outer part of the zone. 



(6.) Place four lighted candles in a dark room before a 

 white surface, and push between the candles and the screen 

 towards the centre of the series an opaque screen e.g., 

 cardboard, with a clean cut vertical edge. A part of the 

 white surface is illuminated by all four candles, then a 

 vertical area illuminated by three, and so on, and finally a 

 part not illuminated by any of the candles. Each of these 

 areas is throughout its entire extent equally illuminated, 

 yet on the side where each area abuts against a darker area 

 it appears lighter, on the other side darker, and gradually 

 shaded between its outer and inner limits. This is due to 

 the fact that strong stimulation of one part of the retina 

 diminishes the excitability in the other parts, and the parts 

 most affected are those next the excited area. Thus a 

 change in the excitability of one part of the retina is brought 

 about by stimulation of an adjacent part. 



