42 COLOUR VISION 



CHAPTER IV 



THE LUMINOSITY OF THE SPECTRUM 



' It is to be noted that the most luminous of the prismatic colours 

 are the yellow and orange. These affect the senses more strongly than 

 all the rest together ; and next to these in strength are the red and green. 

 The blue compared to these is a faint and dark colour, and the indigo 

 and violet are much darker and fainter, so that these compared with the 

 stronger colours are little to be regarded 1 ." 



Fraunhofer 2 first published in 1817 measurements of the brightness 

 of the various parts of the spectrum. Vierordt 3 published a very similar 

 curve of the " strengths of coloured light " of the sun's spectrum. He 

 measured the amounts of the white light which must be added to 

 various parts of the spectrum in order to produce a just appreciable 

 diminution in saturation. Fraunhofer's and Vierordt's curves agree 

 well except towards the violet end, a fact which is of considerable 

 theoretical importance 4 . Draper 5 however found all parts of the dif- 

 fraction spectrum equal by this method, and the red brightest in the 

 prismatic spectrum. Accurate investigations of more importance for 

 our purpose date from those of Abney and Festing 6 and Konig and 

 Ritter 7 , but before entering upon these and other researches we must 

 attack the difficult problem of realising what exactly is being measured. 

 It may be stated at once that we shall be unable to give a completely 

 satisfactory solution to the problem at this stage, but as we proceed in 

 our discussion the profound importance and significance of luminosity 

 in colour vision will become more apparent. 



Two white lights of different intensity impress the senses with 

 stimuli which are interpreted as differing in brightness or luminosity. 

 The measurement of differences of luminosity is the function of photo- 

 metry. 



Weber's and Fechner's laws (p. 20) suffice to show that even with 

 white light the relationship of the sensations to the stimuli is by no means 

 simple. When lights of different colours are compared the difficulties 



1 Sir Isaac Newton, Optics, Bk. r. Part I, Prop, vii, Thcor. vi. 1704. 



2 Gesammelte Schriften, p. 1, Miinchcn, 1888. 



3 Pocjg. Ann. cxxxvn. 200, 1869. 



4 Cf. Helmholtz, Ztsch. f. Psycliol. u. Physiol. d. Hinnesorg. n. 1, 1891. 



5 Lond., Eclin. and Dublin Philos. Mag. vra. 75, 187!). 



u Phil. Trans. Roy. Soc. Lond. 1886, 1888, 1892, 1899, etc. 

 7 1891, in Konig, p. 144. 



