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Show ( 6 ) NOTE II.-PRIMAR Y COLOURS. Cling rou11d ~be aerial how 'with prifms hrig·h~, A~td pleafed unt'lvi.Jl the J evenfold threads of lzght. CANTo!. I. II7· THE · 1 · h the ra ·1nbow is produced was in fomc meafure underll:ood < manner 111 w 11c before Sir Haac N ewton had difcovcred his theory of colours. The firll: perfon who f I ll 1 ewed the rainbow to be formed by the refleetion of the funbeams from drops ex pre s y II ~ I d d'fr ai of falling rain was Antonio de Domin is. This was afterwar.ds more ul Y an I l1l Y I. · db Des Cartes. But what caufeci the diverfity of tts colours was not then un-exp a111e y . f ]' h derHood. it was referved for the immortal Newton tod1fcover that the rays o tg t con-ftfl: cd of 1 feven combined colours of different refrangibility, which could be feparated at pleafure by a wedge of glafs. Pemberton's View of Newton. Sir I faac Newton difcovered that the prifmatic fpeClrum was compofed of feven colours in the following proportions, violet 8o, indigo 40, blue 6o, gre.en 6o, yellow ~8, orange d I f all thefe colours be painted on a circular card 111 the proportiOn above- 2.7, re 45· . . . I mentw· ned , a nd the card be rapidly wlurled on tts center, they produce m. th. e eye t 1e fenfation of white. And any one of thefe colours may be imitated by p~mtmg ~ card with the two colours which are contiguous to it, in the fame proportwns as 111 the fpeCl:rum and whirling them in the fame manner. . . . My ingenious friend, Mr. Galton of Birmingham, afcerta1.ned m th1s m:mner ~y a fet of experiments the following propofitions; the truth of which he had preconcetved from the above data. 1 • Any colour in the prifmatic fpechum may be imitated by a mixture of the two colours contiguous to it. 2 • If any three fucceffive colours in the prifmatic fpecrrum are mixed, they compofe only the fecond or middlemofl: colour. . . . 3 . If any four fucceiTive colours in the prifmatic fpectrum be m1xed, a .tint fimtlar to a mixture of the fecond and third colours will be produced, but not prec1fely the fame becaufe they are not in the fame proportion. 4 . If beginning with any colour in the circular fpeetrum, you take of the fecond colour a quantity equal to the firfl:, fecond, and third; and add to that the fifth colour, equal in quantity to the fourth, fifth, and fixth; and with thefe combine the feventh colour in the proportion it exiHs in the fpechum, white will be produced. Becaufe the firll:, fecond, aml third, compofe only the fecoml; and the fourth, fifth, and fixth, compofe only the fifth ; therefore if the feventh be added, the fame effeet is produced, as if all the [even were employed. 5. Beginning with any colour in the circular fpeCl:rum, if you take a tint compofed of a certain proportion of the fecond and third, (equal in quantity to the firil, fecond, third, and fourth,) and add to this the fixth colour equal in quantity to the fifth, fixth, and feventh, white will be produ~ed. NoTE II. PRIMARY COLOURS. 7 From thefe curious experiments of Mr. Galton many phenomena in the chemical changes of colours may probably become better underll:ood; efpecially if, as I fuppofe, the fame theory mull apply to tranfmitted colours, as to reflected ones. Thus it is welL known, that if the glafs of manganefe, which is a tint probably compofed of violet and in~igo, be mixed in a certain proportion with the glafs of lead, which is yellow, that the mrxture becomes tranfparent. Now from Mr. Galton's experiments it appears, that in reflected coluurs fuch a mixture would produce white, that is, the fame as if all the colours were reflected. And the:cfore in tranfO:itted colours the fame circumftances muftproduce tranfparency, that Is, the fame as If all the colours were tranfmitted. For the particles which confiitute the gbfs of manganefe will tranfmit red, viofet, indigo, and blue; and thofe of the glafs of lead will tranfmit orange, yellow, and green; hence all the primary colours by a mixture of thefe glaffes become tranfmitted, that is, the glafs becomes tranfparent . . Mr: Galton has f~rther obferved that five fucceffive prifmatic colours may be comhl. ned 111 fuch pr~portwns as to produce but one colour, a circumftance which might be of confequence m the art of painting. For if you begin at any part of the circular ~pectr~1m above d.e f~ribed, and take the firll:, fecond, and third colours in the proportions 111 wh.1ch they exlfl: m the fpeCl:rum ; thefe will compofe only the fecond colour equal in quanttty to the firll:, fecond, and third; add to thefe the third, fourth, and fifth in the propo.rtion they ~x.ill: in the fpeClrum, and thefe will produce the fourth colour equal in qu.a~t1ty to the third, fourth, and fifth. Confequently this is precifely the fame thing, as m~xmg the fecond and ~ourth colours only; which mixture wouh.l only produce the tlm~ col~ur .. Therefore.lf .you combine the firfl, fecond, fourth, and fifth in the proportiOns m which they ex1il u1 the fpeCl:rum, with double the quantit'y of the third colour, thi.s thi~d. colour will be produced. It is probable that many of the unexpected <;hanges m mlxmg colours on a painter's pallet, as well as in mor~ fluid chemical mixtures, may depend on thefe principles rather than on a new arrangement or combination 0f their minute particles. . M:. Galton f~rthe~ obferves, that white may univerfally be produced ,by the combma. tl~n of.onepnfmatrc colour, and a tint intermediate to two others. Which tint may be dill:rnglllfhed by a name compounded of the two colours, to which it is intermediate. Thus white is pro~uce~ by ~ ~ixture of red with blue-green. Of orange with indigoblu~. ?f ye~low wtth vtolet-mdtgo. Of green with red-violet. Of blue with orange-red. Of mdrgo wJth yellow-orange. Of violet with green-yellow. Which he further remarks exactly coincides with th.e theory and faCls mentioned by Dr. Robert Darwin of Shrewfbury in his account of ocular fpettra ; who has fhewn that when one of thefe contrafied colours has been long viewed, a fpectrum or appearance of the other becomes vifible in the fatigued ey~. Philof. Tranf. Vol. LXXVI. for the year 1786. ~hefe expenments. of Mr. Galton might much aiTill: the copper-plate printers of cal!Icoes and papers In colours; as three colours or more might be produced by two copper-plates. Thus fuppofe fomc yellow figures were put on by the firH plate, and upon fomc parts of thefe yellow figures and on other parts of the ground blue was laid on by another copper-plate. The three colours of yellow, blue, and green mjght be produced ; as green leaves with yellow and blue flowers. |