354 STUDIES IN SPECIAL SENSE PHYSIOLOGY 



One of the best is the " flicker " method which has been carefully 

 studied in England by Haycraft ( 4 ) and Rivers ( 5 ). If a series of 

 sectors are whirled round on a colour mixer, the rapidity necessary 

 to produce a fused sensation depends upon the brightness of the 

 sectors ; hence with different sets of sectors equal in size, the 

 velocity of rotation which just extinguishes the sensation of flicker 

 will afford us a measure of brightness, this varying, within certain 

 limits, inversely as the rapidity of rotation. Many other plans 

 can be, and have been, adopted, some of which will be mentioned 

 later. The above should give the reader some idea as to how 

 researches can be arranged. 



Hering, in 1890, noted the relative darkening of red as one 

 passes from central to peripheral vision ; he compared a pure red, 

 a spectral mixture of red and blue green (656 /A/A + 470 /A/A) and 

 daylight. The converse was found to hold for spectral green 

 (505 /A/A), but his results were not quite pure, since momentary 

 dark adaptation occurred during the experiments ( 6 ). Tschermak ( 2 ), 

 who studied the whole question systematically in the " light " eye, 

 found with indirect vision a relative diminution in brightness for 

 light of wave length between 693 and 525 /A/A, no change from 

 525 to 516 /A/A, an increase from 516 to 466 /A/A. Similar changes 

 were observable in " dark " eyes. 



Another line of research was to start with a large field of colour- 

 less light, produced by mixing together cornplementaries, and then 

 to diminish its size. It has been found that a colourless mixture 

 of spectral red and bluish-green becomes, with such areal diminu- 

 tion, redder and darker ; if the change of size be an increase, it 

 becomes greener and brighter. In fact, both for " light " and 

 " dark " eyes, colourless matches valid for the periphery do not 

 hold for the centrum and vice versa. Apart from adaptation, it 

 is interesting to see whether merely changing the physical in- 

 tensities of the two mixtures renders the match invalid. It is 

 agreed that mere intensity variation does not affect central matches, 

 but there is much dispute as to the periphery. V. Kries and his 

 fellow-workers have brought forward experimental evidence on 

 the affirmative side, but difference of opinion exists. Thus, 

 v. Kries ( 7 ) found the equation R -f G = Y, no longer valid on 

 diminishing the intensities of both fields, the binary colour growing 

 paler and brighter. We must remember that it is extremely 

 difficult to avoid changes in the state of adaptation, and as these 



