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LXIV. Intelligence and Miscellaneous Articles. 



ON THE VALUE OF THE BRIGHTNESS OF SPECTEAL COLOURS 

 WITH DIFFERENT ABSOLUTE INTENSITY. BY A. KONIG AND 

 R. RITTER. 



[" PURKINJE was the first to point out that the relative bright- 

 ^ • ness of different pigments was unequally influenced by a change 

 in the intensity o£ their illuminations. In twilight blue is brighter 

 than red, even though for mean illumination both colours seem to 

 be equal. Dove and Seebeck subsequently pursued the phenomenon 

 more minutely, and von Helmholtz found that it was also to be 

 observed with spectra] colours. If two differently coloured fields 

 have the same brightness for mean illumination, then, after equal 

 diminution in the objective intensity of both, that field which is 

 illuminated by light of short wave-length will be the brighter, while 

 after an increase of the intensity the longer wave-length seems 

 brighter. Some years ago Brodhun * investigated this " Purkinje's 

 phenomenon " quantitatively, and found that with the illuminations 

 he used it could no longer be observed at higher limits ; that is to 

 say, that beyond a certain illumination all colours alter their relative 

 brightness to the same extent when their objective intensity is 

 increased to the same extent. Only at this illumination are we 

 entitled to represent by a curve the distribution of intensity in the 

 spectrum without any special statement as to the illumination at 

 which the comparisons are made. Brodhun has made and pub- 

 lished such curves for my own eyes (normal trichromatic), for his 

 own (green-blind), and for Eitter's (red-blind). 



It is, however, apparent that curves of the distribution of 

 brightness in the spectrum can be obtained for each degree of 

 brightness ; they possess then no value unless at the same time 

 the degree of brightness is given in such a way that the curve can 

 be reconstructed. 



In a comprehensive research, of only a small portion of which 

 I here give an account f , B,. Bitter and I have endeavoured to ob- 

 tain such curves for our own eyes, and for those of some other 

 observers, at very different brightnesses. In the change of form of 

 these curves with increasing absolute intensity, Purkinje's phe- 

 nomenon must of course come out, so that with increasing bright- 

 ness the ordinates increase the more, the longer is the wave-length 

 of the spectrum-light in question ; by this the maximum is displaced 

 in the direction of the red end. 



* E. Brodhun, Beitrdge zur Farbenlelire : Inaugural diss., Berlin 1887. 



t A. Konig, "Ueber den Helligkeitswerth der Spectralfarben bei 

 verschiedener absoluter Iutensitai." In Beitrdge zur Psychologic und 

 Physiologie der Smnesorgane, von Helniholtz-Fes'tschrift : Hamburg- and 

 Leipzig, 1891 (Leopold Voss). 



Phil. Mag. S. 5. Vol. 33. No. 205. June 1892. 2 



