XXVII.] VISION. 287 



to a yellow surface; there will be visible a 

 green patch. 



e. Look fixedly at a brightly illuminated win- 

 dow 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 suc- 

 ceeded by coloured images. 



18. Contrast. 



Place three candles in front of a white, otherwise 

 un-illuminated surface ; pass between them and 

 the surface an opaque body with a sharp clean- 

 cut edge, so that part of the surface is illuminated 

 by the three candles, part by two, part by one 

 and part un-illuminated; stand two or three 

 yards back and look fixedly at the junction lines 

 of the variously illuminated surfaces ; it will be 

 seen that each area is lighter close to a darker 

 surface and darker close to a lighter one than it 

 is elsewhere. 



19. Simultaneous Contrast. 



a. Cut out a thin cross of grey paper, and place 

 it in the middle of a sheet of bright green 

 paper. Cover the whole with a sheet of thin 

 tissue paper. The grey patch will appear 

 pink. The exact tint of the patch will 

 depend on the tint of the green, of which 

 it will be the complementary colour. 



