their Relation to certain other Visual Phenomena. 



277 



any indication whatever of the colours to which they are due, but it is 

 perhaps even more surprising to find that parts of the retina upon 

 which the intermittent white light does not fall may also be absolutely 

 blind to the exciting colour. 



The effect in question is conveniently demonstrated by the arrange- 

 ment illustrated in fig. 9. A piece of clear glass, upon which is 



Fia. 9. 



gummed a small circle of black paper or tinfoil, is fixed behind the 

 iris-diaphragm P, fig. 3, and thus a round black spot, 0*6 cm. in 

 diameter, is formed at the centre of the white-light disc projected 

 upon the screen L. In fig. 9 the outer circle represents the white- 

 light disc, the shaded circle the colour-patch, and the inner one the 

 black spot upon the white-light disc. Suppose the colour -patch to be 

 green. When the apparatus is worked, the shaded circle becomes 

 purple • the site of the black spot, being illuminated five or six times 

 in a second by green light, might be expected to appear green ; but if 

 viewed from a distance of 30 cm. or more it remains perfectly black 

 throughout ; under normal conditions no trace of a flicker of green 

 light can be seen upon it. The apparent width of the blind region 

 adjoining the site of the pulsative image, therefore, exceeds half a 

 degree. 



This induced blindness is most conspicuous when the light is green, 

 and hardly less so when it is yellow ; it does not occur at all with 

 extreme red nor with violet light, which illuminate the site of the black 

 spot quite strongly; but its absence is certainly not entirely due to 

 the inferior luminosity of those hues. With a very narrow slit green 

 can indeed be seen in the central part of the spot by an observer 

 stationed quite near the screen ; but if he is at a distance of 1*5 metre, 

 the green light may be weakened by gradually closing the slit until 

 the pulsative image completely disappears, yet no green is ever seen 

 upon the spot. 



The following are the results noted in one experiment, when a slit 

 was moved across the spectrum from end to end. Eed was seen upon 

 the spot, at first nearly continuously, then intermittently, until the slit 

 reached about A 6220, when, unless the illumination was made very 



