March ist, 1887.] 



SCIENTIFIC NE\VS. 



13 



names in his "Thesaurus," and perhaps there are not more 

 than two hundred names for colours in the EngHsh language. 

 It has been proposed to make a standard catalogue of colours 

 which could be referred to by manufacturers and dj'ers. 



The second popular error, and one which it is very diffi- 

 cult to eradicate, is that there are three primary colours — 

 red, yellow, and blue. This theory was supported by Sir 

 David Brewster, whose optical discoveries gave him con- 

 siderable claim to be heard. The common argument is, 



of colours proper. There are many ways, but they are 

 beyond the scope of this article, in which it may be plainly 

 shown that blue and yellow, or green and purple, or red and 

 blue-green make white, as red and green make yellow. The 

 red-seeing nerves take in the orange and j'ellow, and the 

 green set takes in the yellow and blue, so that a yellow tint 

 is known by its action on both the red and the green set in 

 a proportion depending on the kind of yellow. When all 

 three are equally excited, the effect of white is produced. 



"You cannot make either of the other colours by any mixture 

 of two of the three colours, while anj' colour can be matched 

 by a suitable mixture of them." The fact is that this power 

 of producing other colours by mixture of paints is due to 

 their not being simple, but compound colours, as has been 

 already explained. 



There is, however, another three-colour theory, which is 

 supported by the names of Young, Maxwell, and Helmholtz, 

 and this is founded not on paints, or indeed on any external 



Fig. 5- 

 colours, but on the structure of the eye. It has been almost 

 indisputably proved that there are three sets of nerves, or 

 three processes of sensation in the eye, for red, green, and 

 blue or violet. The blue may be approximately represented 

 by ultramarine, the red by vermilion, and the green by 

 emerald-green paint, but the two latter are somewhat too 

 light. It is true that these three colours seem ill-suited to 

 make, by their combination, all other colours, but that is be- 

 cause one is accustomed to think of mixtures of paints, not 



If one of these sets ot nerves is fatigued by dwelling for 

 some time on nothing but the colour which principally 

 characterises it, it becomes less capable of being stimulated 

 by it, and if all three sets are then given an equal chance, 

 by looking at a white suiface, the other two sets of nerves 

 being, so to speak, fresher in the field, have it all their own 

 way, and give the impression of a blue-green. An example 

 of this is well known in the form of a popular advertisement. 



Another instance of the fatigue and repose of the different 

 colour-perceptions is to be noticed at night. Artificial light 

 is generally rich in yellow and red rays, and deficient in 

 blue and violet. In fact, after dark the blue-seeing nerves 

 being, as a rule, so little used, the others warp the judg- 

 ment of the mind and give a false standard of white. The 

 standard is yellowish, but it is not as though we see through 

 yellow glasses. The more the nerves are excited the 

 whiter is the effect. The mind becomes accustomed to treat 

 a full excitement of the red, a partial excitement of the 

 green, and a feeble impression on the violet seeing nerves, 

 as an equal stimulation of the three, that is, white ; and 

 when a more liberal proportion of blue rays is present, the 

 effect appears as an excess of blue. Who has not heard 

 the expressions " blue " and " steely " as applied to the elec- 

 tric light, while its colour is really a pale primrose when 

 compared with the light of day ; and the sun itself, which 

 is after all our only ideal of white, is thought by some to be 

 inferior to many stars in the whiteness of its colour. 



The Institution of Civil Engineers. — The number of 

 members, associates, and students of this Institution on the 

 2nd of January, the sixty-ninth anniversary, was as follows: — 

 Members, 1,556; Associate Members, 2,231; Associates, 48S ; 

 Honorary Members, 20 ; and Students, 929. The total was 

 therefore 5,224, representing an increase in the past year at the 

 rate of 5^ per cent., and of more than 100 per cent, since 

 iS75- 



