Jan. 1, 1886.] 



♦ KNOWLEDGE ♦ 



95 



advance, looks back at liis own career, is more apt — if lie is 

 conscientious at all — to feel the reproach of conscience for 

 time given to advance scientific knowledge which should 

 have been devoted to his family, than for labours directed 

 to the well-being of his fikmily when he might have 

 been advancing scientific knowledge. And this point of 

 conscience really settles the question of dignity; for a 

 course can never be con>istent with dignity which is 

 contrary to the dictates of conscience. 



OPTICAL RECREATIONS. 

 Bv A Fklluw of the Royal Astrosomical Society. 



COLOUK AND LIGHT, 

 N speaking of the ultimate constitution of 

 white light on pp. 48 and 49, we said that 

 in decomposing a beam of solar iight by the 

 aid of a prism into five distinct bands of 

 colour, we had " not even yet reduced our 

 spectrum to its lowest terms." Let us 

 endeavour to ascertain how much farther we 

 can simplify it. 



Every possessor of a paint-box knows how very few 

 paints are really needed to represent — at iill events 

 approximately — the majority of colours we meet with in 

 nature. Suppose, for example, that we are restricted to 

 cArmine, Indian yellow, and Prussian blue. The carmine 

 and the Indian yellow, mixed in various proportions, will 

 give us a series of oranges and orange-reds ; the Indian 

 yellow and the Prussian blae a large range of greens : 

 while all sorts of violet and purple hues are obtainable 

 from varying mixtures of Prussian blue and car- 

 mine. This, the employment of an impure spectrum, 

 i.e., one mixed with white light, and some misun- 

 derstood experiments with absorptive coloured media, 

 induced Sir David Brewster to propound the theory 

 in his work on " Optics " in " Lardner's Cabinet 

 Cyclopaedia '' (pp. 72 it seq.), that white light 

 has in reality but three primary constituents — red, 

 yellow, and blue. He announced this in the work just 

 cited as an original and independent discovery ; but he 

 had been long anticipated in his doctrine of the triune 

 chariicter of light, first by Wiinsch at Leipzig in 1792, 

 and not long subsequently, at the beginning of the 

 present century, by that truly great philosopher and 

 phj-sicist Thomas Young, in this country. Nay more : so 

 far from Brewster having made a discovery, he absolutely 

 enunciated an erroneous doctrine, and took a retrograde 

 step in comparison with those of his predecessors. It is 

 quite true that the light of the sun, as it reaches us, is 

 compounded of three colours, and of three only : but un- 

 fortunately two out of Brewster's three colours happen 

 to be the wrong ones ! For while it is the veriest 

 truism to assert that blue and yellow paint mixed together 

 form green, no such result can be predicated of the mix- 

 tui'e of blue and j-ellow light ; which mixture, so far from 

 being green, is white. These colours are. in fact, comple- 

 mentary to each other. Helmliolt/, has explained how it 

 is that blue and yellow pigments produce green by admix- 

 ture. If we make solutions of, say, Prussian blue and 

 gamboge, we do not obtain a pure blue, or a pure yellow. 

 In the former case a quantity of green light gets 

 through with the blue ; as in jioiut of fact, it 

 does with the yellow light too. Then sup)posing 

 that we mix such a blue and yellow as this, the blue will 

 . cut ofi the yellow, the orange, and the red end of the 

 spectrum ; and the yellow will similarly obliterate the 

 blue, indigo, and violet end. VTe have seen, though, 



that both are transparent to green, so that when 

 white light passes through a mixture of blue and 

 yellow paint, or through superposed blue and 3'ellow glass, 

 the green alone reaches the eye. Furthermore, if we 

 carefully select colours of the proper degree of refran- 

 gibility, and paint discs with them, causing the light 

 reflected from these discs (by a contrivance to be imme- 

 diately adverted to) to reach the eye simultaneously, by 

 no artifice whatever shall we succeed in producing a green 

 hue, or, in fact, anything but a greyish white. The fact 

 is, as we shall see by and bye, that the three primitive 

 colours are a scarlet red, emerald green, and violet blite, 

 or blue violet. 



For the purpose of compounding colotu'ed lights, what 

 are known as " Maxwell's discs " (from their inventor, the 

 late lamented Clerk-Maxwell) aSord considerable facili- 

 ties. They are simply circular discs of white cardboard, as 



B 



Fig. 6. 



shown in Fig. 6, A, each furnished with a radial slit which 

 enables two or more of them, each being painted of the re- 

 quired colour, to be superposed on the spindle of an ordinary 

 whirling table. B in the fignre shows two of these discs as 

 being partlj- superposed. A glance at the figure — or, better 

 still, the cutting out of two such discs, will show how 

 the proportion of any given colour to any other can be 

 obtained by simply twisting one of the discs round its 

 own centre. The whirling table scarcely demands de- 

 scription here; but we may mention that the one 

 employed by ourselves for the purposes of this series of 

 papers consists ( f a base-board 36 inches long, 8 inches 

 wide, c-.nd i; inch thick, at one end of which a grooved 

 wheel 9 inches in diameter rotates on a vertical spindle, 

 and, by the aid of a jiiece of fine cord, drives another 

 small vertical spindle at the other end of the board, the 

 last-named (me carrying the discs which we have jtist 



l-'i?- '• 



been describing. If now we jniint two of these discs 

 with chrome yellow and ultramarine, fasten them on our 

 spindle and set them in rapid rotation, we may vary the 

 area of blue or yellow respectively in any propc^rtion 

 we please, but we shall never obtain any resultant tint in 

 the smallest degree resembling green. 



Another simple piece of apparatus for mixing coloui-ed 

 light is shown in Fig. 7, and may be bought at any of the so- 



