STUDIES IN SPECIAL SENSE PHYSIOLOGY 355 



are especially important at the periphery, it is probable that 

 some of the results have been vitiated by this circumstance 

 (Tschermak 2 ). 



In connection with the supposed relation between physical 

 intensity and apparent brightness, an interesting set of observa- 

 tions may be noted regarding what is called the " achromatic 

 threshold of coloured light." In studying Purkinje's " phenomenon " 

 we found that if the intensity of a spectrum were steadily diminished, 

 a point was reached at which the brightest region appeared to have 

 been moved towards the violet. What, it may be asked, happens 

 if the illumination be still further diminished ? Under normal 

 circumstances we soon reach a point at which the whole spectrum 

 appears colourless, differing, however, in brightness in the various 

 regions. Reduction beyond this yields an intensity which is 

 associated with no sensation at all. Hence it seemed necessary 

 to distinguish between the absolute liminal intensity of a spectrum, 1 

 that is, the least intensity corresponding to a colourless sensation, 

 and the " specific " threshold or liminal value for which the spectral 

 zones could be seen to differ in hue. 



A good many workers, including Purkinje, Helmholtz, Aubert, 

 Landolt, &c., observed this phenomenon, and the actual absolute 

 threshold values for various spectral lights and different parts of 

 the retina have been studied. Charpentier ( l ) found the " photo- 

 chromatic interval " (i.e. the difference between specific and abso- 

 lute threshold intensities) least for red and greatest for blue light, 

 and that the absolute thresholds were diminished by dark adapta- 

 tion, while the specific liminal values were not affected. It also 

 appeared that these results were more noticeable with peripheral 

 than central vision. 



It was soon noticed, however, that the phenomena were in 

 reality complex. Some workers (e.g. Parinaud and v. Kries) 

 never obtained an achromatic value for red, and doubts arose as 

 to the existence of such intervals at all. Finally, on comparing 

 the brightness of central and peripheral lights, rendered colourless 

 by a reduction of physical intensity, unexpected and puzzling 

 results were obtained. 



Stegmann ( 8 ) performed the following experiments. Matches 

 were made between a series of lights from 640 to 480 /*M, and a 



1 The liminal value or threshold value of a stimulus is that intensity which 

 corresiKmds to a "just- noticeable " sensation. 



