STUDIES IN SPECIAL SENSE PHYSIOLOGY 409 



including the phenomena of after-images ; that it also describes 

 the facts of partial colour-blindness adequately enough, but in 

 very general terms, the earlier direct explanation being insufficient. 

 We have now to see whether any experimental evidence can be 

 found pointing to the existence of independent visual components 

 satisfying the above conditions, or conceivably capable of satis- 

 fying them. 



Evidence of this kind is furnished by the experimental pro- 

 duction of a peculiar form of colour-blindness by G. J. Burch. ( 18 ) 

 This observer exposed his eye to direct sunlight in the focus of 

 a two-inch lens behind coloured glasses. A gelatine film stained 

 with magenta and combined with a medium ruby glass was found 

 to transmit a fairly pure red, three thicknesses of green glass 

 were used for green, and a tank of cupric ammonia-sulphate for 

 violet. Similar arrangements were made for the other hues, and 

 in some experiments a large spectroscope was employed. Two 

 minutes' exposure was sufficient to produce the maximal effect in 

 the case of red. 



After exposure to red light, the following effects were noted. 

 Scarlet geraniums appeared black, calceolarias and sunflowers green, 

 purple flowers, such as clematis, violet. Pink roses were sky blue. 

 Fatiguing with violet light caused objects reflecting violet light 

 to appear black, purples and reds seemed crimson. Green stimu- 

 lation made the foliage appear reddish or bluish-grey. But, after 

 these exposures, " the colour by which the eye has been dazzled, 

 and to which it is now blind, tints all those objects which naturally 

 reflect none of this." The truth of this is illustrated by a simple 

 experiment. ( 19 ) Suppose the eye to be somewhat fatigued by green, 

 as during a long country walk, if under these circumstances the 

 eye be directed to a small red spot on a black surface, e.g. a 

 geranium petal on the black cover of a book, and one walks 

 quickly with it into a dark shed or barn, the colour of the petal 

 changes from red through orange and yellow, becoming eventually 

 perhaps whitish. On coming into the light again the red re- 

 appears. 



These experiments suggest that stimulation with red, green, 

 and violet alters responsiveness with respect to these stimuli alone, 

 and the same is found in the case of blue. Orange stimulation, 

 on the other hand, affects not only the appearance of the orange 

 but that of the red and green as well. Both positive and negative 



