96 



♦ KNOWLEDGE 



[Jan. 1, 1886. 



called " uoveltj- shops " in London. Its primary purpose 

 is to teach drawing by enabling a child to trace on paper 

 behind a reflected image. It is, however, equally useful 

 for our present purpose. It consists of a thin flat base, 

 from which rises centrally, and at right angles, a plate of 

 glass, G. The base must be blackened with a dead black, 

 either by pasting a piece of such paper over it, or by 

 painting it with lampblack and size. If now we place a 

 piece of coloured jiaper (say blue) at B, and another 

 (suppose yellow) at Y; then, the eye at E will see the 

 former directly through the glass and the latter by re- 

 flection seemingly superposed on it, in a wr,y which our 

 figure above will render immediately intelligible. If it 

 be desired to vary the brightness of the image, this may 

 be eilected by moving the bits of coloured paper further 

 apart or nearer together as the c;ise may be. 



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Fig. 8. 



Yet another and verj- effective method of mixino- 

 coloured light is .<hown in Fig. 8, when C is a piece of black 

 cardboard in which two circular holes are punched with an 

 ordinary gun- wad punch, with an interval of -' inch or so 

 between their peripheries, and then are covered with thin 

 sheets of gelatine of the required hues — let us say in this 

 case red and green. P is a common " double image prism," 

 ■purchasable of all dealers in polarising apparatus. The 

 familiar effect of this is, of course, to duplicate all imace.s, 

 and so cause an apparent overlapping of the colours, the 

 prism being rotated until superposition occui-s and a 

 suflicient area of the combined lights becomes visible. 

 Should we chance to hit upon the two proper colours, 

 the overlapping portion will approximate to white. We 

 shall, though, most probably find that it will be yellowish 

 or orange, thus definitely exploding the idea of Sir 

 David Brewster that yellow is a simple colour. Or, 

 again, by the aid of two magic-lanterns such as are used 

 for dissolving views (and preferably illuminated by the 

 electric or the lime-light), we may superpose' the 

 images of two coloured discs of gelatine upon a white 

 screen, so as to exhibit to a large number of people at 

 once, in a most striking and instructive form, the 

 effect of the mixture of the primary 

 spectral colours. Lastly, by a very 

 simple little artifice we maj' mLx 

 the colours cf the spectrum them- 

 selves. All we have to do is to 

 cut two very narrow slits in a black 

 card-— or, perhaps preferably, a 

 sheet of latten-brass — as shown in 

 Fig. 9, and view them through 

 a prism whose edge is parallel 

 to their length, from a distance of a yard or so. 

 It will easily be seen how and why the spectra 

 will overlap. It is needless to add that the train of 

 prisms in a " direct-vision " spectroscope exhibits this 

 effect in a much more striking form. We would 

 earnestly urge on the student the necessity of rc- 



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peating every one of these experiments for himself. He 

 will thus realise their import in a manner wholly un- 

 attainable from the mere perusal of our descriptions 

 AYhichever of the modes just described we adopt for 

 combining coloured lights, we shall find that red, mixed 

 with greenish blue, orange and a greenish blue, bright 

 blue and bright yellow, and greenish yellow and violet 

 light, when mixed in the pairs indicated, all four give 

 white light as a result. In the case of Maxwell's discs, 

 the resulting hue is always more or less grey, rather than 

 white. The origin of this lies in the difference of lumi- 

 nosity of the two colours, and it is quite possible upon a 

 few trials accurately to imitate the very pale grey fur- 

 nished by (say) a pair of blue and yellow discs, b}- means of 

 a properly proportioned black and white pair alone. Now" 

 it was upon researches into this elementary constitution 

 of light that Young founded his famous theory of vision, 

 more recently adopted and expounded by Helmholtz. 

 He supposed that each element of the retina of the 

 human eye (Fig. 1, p. 16) possessed the capacity for 

 transmitting three kinds of sensations ; or, to put it iu 

 another way, each element was entered by three fibrilla- 

 of a nerve, one of which vibrated to, or was excited by, 

 red light, the second in like manner h\ green light, and 

 the third by violet light. It would be more rigidly 

 correct to siy that these fibrilhv are principalltj excited 

 by light of the colours named, respectively ; since Young 

 conceived that what we may call the red nerve fibril 

 is very feebly responsive to green and violet waves 

 as well ; the green fibril to red and violet waves, and 

 the violet fibril to red and green waves ; albeit the 

 overwhelming part of their excitation proceeds from 

 those coloui's alone to which each is more especially 

 adapted. When the three sets of nerves are equally 

 "excited we experience the sensation which we call white. 

 When the set most sensitive to the least refrangible rays 

 is excited, we see red; when the nerves affected by the 

 middle of the spectrum are set in motion we see green : 

 while the nerves most susceptible to the most refrangible 

 end of the spectrum when excited give us the sensation 

 of blue or violet. As far as the mammalia are concerned, 

 dissection of the eye has, so far, furnished no corrobora- 

 tion of this theory ; but Max Schultze actuallj^ has dis- 

 covered in the eyes of some birds and reptiles, among the 

 rods of the retina many which contain a drop of red oil 

 at the end fronting the cornea or which is turned towards 

 thelig-ht; while other rods contain a yellow drojj, and 

 others none at all. It is not diflicult to see the bearing 

 of this on the theory of Young and Helmholtz. Now 

 assuming this theory to be correct, let us see how it 

 explains the production of the sensations of single colours 

 by the mixture of two others. In the ordinary sptctrum 

 (Fig. 2, p. 16) yellow lies between green and red ; hence 

 a mixture of red light and green light ought to produce 

 the sensation of yellow light by stimulating what we have 

 called the red nerves and the green nerves. This may, 

 perhaps, be most successfully effected in the modes illus- 

 trated by Figs. 8 and 9 above ; but a very tolerable 

 though somewhat dingy yellow is producible by the 

 combination of two of Maxwell's discs (Fig. 6) coloured 

 with emerald green and vermilion respectively, and set 

 in rotation on the spindle of a whirling table. We have 

 said that the resulting tint, will be that of a "dingy 

 yellow ' ; nor is the reason far to seek : the chrome 

 yellow, Indian yellow; or Cadmium yellow of our paint- 

 boxes are very much more luminous than our vermilion, 

 carmine, emerald, or sap green, and it is obviously im- 

 possible, to use a homely form of phraseology, to gel 

 more luminosity out of a combination of colotired discs 



