THE LUMINOSITY OF THE SPECTRUM 43 



are greatly increased. The comparison of brightness or luminosity 

 of coloured lights is known as " heterochromic photometry " (Briicke). 



In conformity with Newton's remarks, quoted at the beginning of 

 this chapter, and with the general consensus of opinion, brightness 

 must be regarded as a quantitative conception. When one colour is 

 added to another on the same field we are conscious that the brightness 

 of the field is greater than with either colour separately. An exact 

 match between two colour mixtures means that they have the same 

 hue and the same luminosity, and v. Helmholtz draws the conclusion 

 that Grassmann's second law (p. 33) can be extended to include 

 brightness 1 . He does not admit, however, that " equally bright lights 

 added give equally bright mixtures," and there is no doubt that 

 this statement is incorrect under certain circumstances (Purkinje's 

 phenomenon, v. p. 57). Apparently this is not the only exception 

 v. Helmholtz would admit, for as he says " equally bright red or blue 

 doubled give the red brighter than the blue." Briickner 2 also holds 

 that the brightness of a mixture is not always equal to the sum of the 

 luminosities of its components, blue and yellow especially giving a 

 brighter mixture. 



v. Helmholtz indeed expresses quite candidly his doubts as to the 

 accuracy of direct comparison of the luminosities of different coloured 

 areas. " I scarcely trust my judgement upon the equivalence of the 

 heterochromic brightnesses, at any rate upon greater and smaller in 

 extreme cases. I admit, however, that one can gradually so darken one 

 of two coloured fields that no doubt remains as to the other being now 

 the brighter.". .." As far as my own senses are concerned I have the 

 impression that in heterochromic luminosity equations it is not a question 

 of the comparison of one magnitude, but of the combination of two, 

 brightness and colour-glow (Farbenylufh), for which I do not know how 

 to form any simple sum, and which too I cannot further define in 

 scientific terms." 



v. Kries 3 says that " it must not be considered as an obvious fact 

 that e.g. a given blue and a given red sensation are necessarily in the 

 strictest sense equally bright. For we could only state this definitely 

 if we were certain that the equality of the luminosity impression 

 corresponded with a particular physiological condition so that we 

 could substitute a well defined physiological entity for the subjective 

 conception of brightness ; but this is by no means the case." 



1 Helmholtz, 2nd ed., p. 440. 2 Arch./. <I. r/r.v. Plttj^iol. xrvm. 90. 1904. 



3 Nagcl's Hanilb. <L Physio!, if. M< '/!<, in. 259. 



