STUDIES IN SPECIAL SENSE PHYSIOLOGY 353 



probable that these changes are due to adaptation. Two rooms 

 which could be independently darkened are separated by a light- 

 proof partition in which two slits are cut and covered with pig- 

 mented glass. The amount of light transmitted by these slits 

 can be varied independently with the aid of reflectors. So long as 

 the room occupied by the observer is kept at a constant illumina- 

 tion, diminishing the physical intensity of the light traversing the 

 two slits, which are covered with blue and red glass respectively, 

 does not change their relative intensities. If, however, the ob- 

 servation room is darkened, the blue slit immediately appears 

 brighter than the red one, even while the physical intensity of light 

 passing through them is unaltered. This effect is enhanced by a 

 prolonged stay in darkness, and is more noticeable in indirect 

 (peripheral) than direct (central or foveal) vision. Burch, in a 

 series of experiments which I shall discuss later, has obtained 

 similar results, so that we may conclude that Purkinje's " pheno- 

 menon " depends not on physical intensity but visual adaptation, 

 and is a particular case of the general change already mentioned. 

 These facts naturally incline us to examine more closely the 

 differences between peripheral and central vision, and we shall 

 find that all work tends to show that the former is more susceptible 

 to adaptive changes and characterised by a heightened responsive- 

 ness to feeble stimuli. 



Before going further, however, we must ask ourselves what is 

 meant by saying that different colours are equally or unequally 

 bright. As a matter of fact, this question is more easi.ly asked 

 than answered. A similar difficulty is experienced in attempting 

 to ascertain what one means by saying that two notes of different 

 pitch are unequally loud. If we attempt to define our meaning in 

 terms of the physical properties of exciting stimuli, we become 

 confused, and perhaps the only valid excuse for employing the 

 expression is an empirical one. Ask a dozen normal persons to 

 look at a spectrum in daylight, and they will agree that the yellow 

 is the brightest part of it, meaning, I suppose, that the region in 

 question produces, somehow, a predominant effect in consciousness. 

 This uniformity of results is a justification, perhaps the only justi- 

 fication, of the method, and allows us to compare different colours 

 with respect to brightness. 



Bearing in mind the empirical and sensational nature of the 

 investigation, we can attain comparative results in numerous ways. 



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