544 Intelligence and Miscellaneous Articles. 



If both are red, the brighter will appear scarlet ; 



„ „ green „ ,, yellowisli ; 



„ „ blue „ „ greenish ; 



,, „ violet „ ,, blue. 



If we have the means of varying the wave-lea gth of the light 

 which illuminates the fainter rectangle, we can improve the match 

 between the two, by bringing the fainter toward the yellow. 

 Such motions will converge toward a certain point of the spectrum 

 which they will never cross — a point a little more refrangible than 

 D and having a wave-length of 582. 10~ 6 mm., according to 



o 



Angstrom's map. If both rectangles be illuminated with this light, 

 the fainter appears white or even violet ; but if it be varied in wave- 

 length with a view of improving the match, it will be found to 

 return to the same point with the utmost precision. 



It appears, therefore, that, if our hypotheses are correct, the 

 colour log r (I i + Jj+TSLk) is like that of the spectrum at \=582, 

 only that it contains less blue or violet, and is consequently of 

 greater chromatic intensity. 



It further follows from Eechner's law that, if any light be gra- 

 dually reduced in brightness, one element of the sensation will 

 disappear after another — and that when very faint it will exhibit 

 only one primary colour, which is the one which it contains in 

 greatest proportion relatively to the proportion in the light which 

 has the colour of brightness, Now although this does not seem 

 to be exactly the case, yet we do get some approximation to it. 

 It is true that any light whatever, when sufficiently faint, appears 

 white, owing to the self-luminosity of the retina. We cannot 

 therefore, unfortunately, get sight of the primary colours by re- 

 ducing the light of three parts of the spectrum. But we may, as 

 has often been suggested, make use of the principle of contrast. 

 If any red spectral light be sufficiently reduced, it will perfectly 

 match any less-refrangible light. We may therefore say that a 

 faint spectral red in contrast with a bright light of the same kind, 

 excites with approximate purity one of the elementary sensations. 

 The same thing is true of the violet ; and therefore a rich violet 

 may be taken as another primary colour. In my book entitled 

 'Photometric Researches,' the printing of which is nearly complete, 

 I show reason to think that the pure green has a wave-length 

 intermediate between E and b. A faint green of this sort contrasted 

 with a bright one appears as a very bluish green ; and this may 

 therefore be supposed to be the third primary colour. 



We have seen that it results from the theory that an increase 

 in the brilliancy of any light adds to the sensation nothing of the 

 peculiar colour of that light, but only a certain amount of the 

 colour of brightness. If this be the fact, then the photometric 

 sensibility of the eye should be the same for all colours. In order 

 to ascertain whether this is so or not, I have made a series of de- 

 terminations of my photometric probable error. Each determina- 

 tion was based on twenty-eight comparisons of two parts of the 

 same-coloured disk. Since there w T ere two unknown quantities 



