122 ELEMENTARY PHYSIOLOGY. XXIII. 



seconds by removing the screen; A states what colour 

 he has seen; it will probably be greyish. 



The observation is repeated, putting the disc 5 nearer 

 the centre each time till A sees a distinct red. The red 

 is then taken back 10 to 15 and shown again for a 

 moment to make certain that it only appears greyish; 

 then a disc of blue is shown, it will probably be seen as 

 a brilliant blue. 



The other discs are then shown in succession; prob- 

 ably green will appear greyish, whilst white and yellow 

 will be recognised at once as white and yellow. 



B then takes the white, yellow and blue discs, and 

 commencing at the periphery shows them in irregular 

 order, till the angle at which the three colours are dis- 

 tinctly recognised is approximately determined. 



Similar observations should be made showing the discs against 

 the lower quarter circle. 



Then a yellowish-green disc and a purple disc should be taken, 

 and shown, beginning at the periphery, they may at first appear 

 grey, but soon the former will appear yellow, and the latter blue. 



15. After-images. On a table in the dark room is 

 placed an electric hand lamp, and a small tracing paper 

 screen behind which stands a lighted candle. The lamp 

 is connected to the electric light circuit and the switch 

 is controlled by A. B seats himself so that he can 

 readily see either the lamp or the tracing paper screen 

 by rotating his head. The lamp is switched on and B 

 gazes at it for 10 seconds without moving his eyes. The 

 lamp is then switched off and B at once looks steadily 

 at the centre of the tracing paper screen. After a few 

 moments a dark image of the lamp filaments will be 



