49C 



KNOWLEDGE • 



[Ai'kiL 7, 1882. 



DOES THE LrMIXOL'S MIXTURE OF BLUE 

 AND YELLOW MAKE CKEEN LIGUTI 



U\ l.iKrr..Coi.<iN»:i. W. A. Komi, latk H.A. 



PnoFKSSOU IIKI.MIIOLT/ iinjii in a lorturo on " Tlio Roront 

 I'ni^rrfmi of tlio Theory of Viiiiun " (Scrond Knglinh cclilioii, 

 IrfinKi"""". I^'^l)- " It ■" iniiKwaiblp to mnki- n );n-i-n out of bliio 

 ami tpIIi'W li|(lit." Ilo iinp|K)rtii this RtronK Hlntcmont by tlio fol- 

 lowiiiK nivumcnt. "Tbo niniplost wny of niixin({* colonrod liK''t." 

 in by tlio oluii-ncr lookin); nl a blue (or yellow) wnfer placed on a 

 table, thn)ii({li n snmll (liit piece of kIo"", "o fixe<l u» to niflo reeeiiT 

 tlio relle»-te<l imin{<< of iinother yellow (or blue) wafer u little (lis- 

 tnnce nwiiy. If I' is the wiifiT from which rays are transmitted 

 thruu)(h the |;la(8, and y. that from which rays are reflected, and 

 if they an> pro|ierly placed, " it then apiicars as if there was a sinfrle 

 wafer at b, with a colour pro<luced by the mixture of the two real 

 ones. In this ex|ieriment, the liffht fnim h which traverses the 

 ulass pane, actually unites with that from J which is reflected from 

 it, and the two combined |ias8 on to the retina." In this way Prof, 

 llelniholtz finds that "the union of blue and yellow rays of light 

 pnHluces white." 



Similarly, Prof. Rood says (" Modem Chromatics," 1879, p. 109) 

 •' Un>wster's theory of the existence of three fundamental kinds of 

 li(tht— red, yellow, and blue — is found in all except the most recent 

 text-books on physics, and is almost universally believed by artists. 

 Nevertheless, it will not bo difficult to show that it is quite without 

 foundation." Eliminating; a rather silly observation, that the theory 

 " cannot be true, because there is no such thing as colour, which is 

 a mere sensation ^-orying with the length of the wave producing 

 it," — on which basis of " argument " wc might ask why Prof. Rood 

 took the trouble to write a book about mixing colours, when " there 

 are no such things" — the "rudo" assertion is found to be sup- 

 ported by evidence <lerived from the same kind of experiment as 

 that of lielniholtz (devised by Lambert) above quoted. 



Now, I do not take upon myself for a moment to assert that the 

 "theory" of blue and yellow lights together forming white may 

 not bo correct ; all I rcspcctfull}- maintain is, that facts, so far as 

 1 have been able to produce them, seem to point in the other, or 

 Browsterian, direction. 



Fact 1. If we make a paste with a drop of distilled water, a 

 little powdcrc<l Alacantite (or pure chloride of copper) and a certain 

 proportion, which can be easily ascertained by experiment, of any 

 sodium salt, and bum this paste on a platinum wire before a blow- 

 pipe, we obtain a beautiful deep green flame, or coloured light ; the 

 green colour being evidently due to the mixture of the blue light 

 from the burning Atacamite, and the yellow light from the burning 

 soda. 



Fact 2. Bend about a quarter of an ir.ch of thickish, clean, new 

 platinum wire at a right-angle to the rest of the wire or shank, and 

 hold it before the blowpipe, so that the yellow flame (afforded by 

 all platinum wires in such a position until they become white hot) 

 shall combine in a certain jiroportion with the blue, blowpipe-pyro- 

 conc, and you immeiiiutely obtain a distinct, though not very 

 " atrong," — from the minute (jnantities of the ingredients used — 

 green light. I ascertained this fact some years ago from some 

 platinum wire given to me by Messrs. Griffin & Co. to examine, as 

 they supposed, from this "green flame," it contained copper; but 

 I showed them that, not only their wire, but everybody's, could, by 

 the above-mentioned manipulation, be made to yield a " green 

 flame." 



Fact 3. Chemists are aware that, by turning a gas — " Bnnsen " 

 — very low, the blue pyrocone suddenly becomes green, just before 

 the little explosion takes place which extinguishes it. The cause of 

 this green colour is soon ascertained to bo the burning of a certain 

 proportion of " sodium " (contained in the air admitted) to the blue 

 Bunnon-pyrocone ; for, if we admit an excess of air to the 

 " Hansen" in full power, we find the blue "flame" almost com- 

 pletely yellowed. There ore other ways of ])roducing a green light 

 from the mixture of blue and yellow lights by direct combustion ; 

 B«, for instance, by introducing a rather moist piece of wood into a 

 cool fire " burning blue," as a friend and I deliberately effected 

 the other nii;hl at his house in Pliillimore-gardens ; but the above 

 will suffice, do far, to disprove Ilelmholtz's assertion. 



Fact 4. Let us now focus, in a semi-darkened room with open 

 window, but Venetian blind down, a sunbeam admitted through an 

 uptamol lath, by means of a biggish lens (I used the object-glass 

 of a lorgnette of 2 in. diameter, when I discovered the fact last 

 <1ctol>er) U|M)n the face of a glass prism held perpendicular to tho 

 lH?ani. Instead of a spectrum on the wall, which the beam, without 



• Qnery : Docs this process really " mix " lights ? 1 should like 

 to SCO this proved. 



the interposition of the Innii, orcoJiions, wc find the sunbeam con- 

 tinued within tho prism, but of a brilliant gram green colour; 

 although bubbles and other objects in the path of the green Ijcam 

 through the glass, reflect white light. This curious effect seems to 

 ine to be the result of the elimination of the least refrangible, or 

 reil mys, from tho beam in its passage thmugh the glass (for ony- 

 sliapcd, if mo<lemtely deep, piece of glass will do, but a mirror will 

 not do), liecause, from their less elasticity when thus concentrated 

 by tho lens on the face of the refracting medium, they refuse to bo 

 bent like the other more refrangible rays (blue and yellow), and 

 thus take up a different path from those ; i.e., are reflected and dis- 

 persed. The blue and yellow rays, left to pursue their path througk 

 the glass alone, combine to form green. 



Your knowledge of mathematical optics will enable yon to detei^ 

 mine whether my ignorance of that subject is causing me to " talk 

 nonsense " or not ; but the above is the only explanation which 

 occurred to mo of this remarkable phenomenon when I observed 

 it last October. Anyhow, the fact is, I believe, quite a new 

 fact in optics ; and, as the same jihenomcnon may occur when- 

 ever solar rays are collected by a lens for delivery upon a 

 prism, as in most spectroscopes, although not with sufficient in- 

 tensity to make it visible, as in the ease of focussing, it would be 

 very interesting to ascertain, by some optical analyzing process, 

 whether the fact may not have affected spectroscopic results as now 

 received ? 



Viewed from Top. 



From Front. 



In these rough figures, tho small white circle lepresents tlio 

 focussed sunbeam impinging upon the face of a glass prism in 

 various positions. The shaded lines show the poth of the beam, 

 now changed to a green colour, within the prism. The dotted lines 

 are intended to represent those sides of the prism which are seen 

 thrnujh it. 



A common glass globular paper-weight is an excellent medium 

 for the exhibition of this phcnomenoD. 



Pojtd's Extract \9 » c«rt*in 

 Pond's Kxtrapt is a c<»r1«in ci 

 Pon.V« ?:xtr«cl i» a (HTliiin ci 

 Fond'a E\tr>rt will hval Bun 

 PoDd'i EilrKt will cure Sprains and BniisM. 



Sold bj all Chemiels. Get tbe genuine. 



re for Rheamatinn and Govt. 



for Hirmorrhoida. 



for Xeiiralpic pains, 

 and Wounds. 



