166 DYEING 



Colours arc not, properly speaking, material ; they are impressions which wo receive 

 from the rays of light reflected, in a decomposed state, by the surfaces of bodies. It 

 is well known that a white sunbeam consists of an indeterminate number of differently- 

 coloured rays, which, being separated by the refractive force of a glass prism, form the 

 solar spectrum, an imago divided by Newton into seven sorts of rays : tin- iv<l, orange, 

 yellow, green, blue, indigo, and violet. Hence, when an opaque body appears coloured, 

 for example, rod, we say that it reflects the red rays only, or in greatest abundance, 

 mixed with more or less of the white beam, which has escaped decomposition. Accord- 

 ing to this manner of viewing the colouring principle, the art of dyeing con 

 fixing upon stuffs, by means of corpuscular attraction, substances which act upon light 

 in a different manner from the surfaces of the stuffs themselves. The dyer ouglit, 

 therefore, to be familiar with two principles of optics ; the first relatively to the 

 mixture of colours, and the second to their simultaneous contrast. 



Whenever the different coloured rays, which have been separated by the prism, aro 

 totally reunited, they reproduce white light. It is evident, that in this composition 

 of light, if some rays were left out, or if the coloured rays be not in a certain propor- 

 tion, we should not have white light, but light of a certain colour. For example : If 

 wo separate the red rays from the light decomposed by a prism, the remaining coloured 

 rays will form by their combination a peculiar bluish green. If wo separate in like 

 manner the orange rays, the remaining coloured rays will form by their combination 

 a blue colour. If we separate from the decomposed prismatic light the rays of greenish 

 yellow, the remaining coloured rays will form a violet. And if we separate tho rays 

 of yellow bordering on orange, the remaining coloured rays will form by their union 

 an indigo colour. 



Thus we see that every coloured light has such a relation with another coloured 

 light that, by uniting the first with the second, we reproduce white light ; a relation 

 which we express by saying that the one is the complement of the other. In this 

 sense, red is the complementary colour of bluish green ; orange, of blue ; greenish 

 yellow, of violet ; and orange yellow, of indigo. If we mix the yellow ray with the 

 red, we produce orange ; the blue ray with the yellow, we produce a neutral tint ; and 

 the blue with the red, we produce violet or indigo, according as there is more or less red 

 relatively to the blue. But these tints are distinguishable from tho orange, green, 

 indigo, and violet of the solar spectrum, because when viewed through the prism they 

 are reduced to their elementary component colours. 



If the dyer tries to realise the preceding results by the mixture of dyes, he will 

 succeed only with a certain number of them. Thus, with red and yellow he can make 

 orange ; with blue and yellow, green ; with blue and red, indigo or violet. These 

 facts, tho results of practice, have led him to the conclusion that there are only three 

 primitive colours : the red, yellow, and blue. If ho attempts to make a white, by 

 applying red, yellow, and blue dyes in certain quantities to a white stuff, in imitation 

 of the philosopher's experiment on the synthesis of the sunbeam, far from succeeding, 

 he will deviate still further from his purpose, and the stuff will by these dyes become 

 coloured of a depth varying according to the quality of the stuff used ; until a full 

 black is produced. Nevertheless, the principle is applicable,, and in many cases 

 adopted in practice by blending the yellow, red, and blue rays in order to produce or 

 improve an otherwise imperfect white. When a little ultramarine, cobalt blue, 

 prussian blue, or indigo is applied to bleached goods with the view of giving them tho 

 best possible white, if only a certain proportion be used, the goods will appear whiter 

 after this addition than before it. In this case the violet blue forms with tho brown 

 yellow of the goods a mixture tending to white, or less coloured than the yellow of 

 the goods and the blue separately were. For the same reason a mixture of prussian 

 blue and cochineal pink, or archil and cudbear, is used for whitening of silks in 

 preference to a pure blue, for on examining closely the colour of the silk to be neutra- 

 lised, it was found by tho relations of the complementary colours, that tho violet was 

 more suitable than the pure blue alono. The dyer should know, that when ho applies 

 several different colouring matters to stuff, as yellow and blue separately, t!i 

 appear green, not because tho colouring matters Jiave combined, but because the eye 

 cannot distinguish the points which reflect tho yellow from .those which reflect t in- 

 blue, and it is this want of distinction that produces tho combined colour. With such 

 a dye the colour will appear of different tints, tho blue or yellow prevailing according 

 to the position in which it is placed to tho eye, whether seen by reflected or trans- 

 mitted light, but when the dye applied to tho stuff is in chemical union, producing a 

 green, such as arsenite of copper, tho yellow and blue rays cannot bo thus dis- 

 tinguished. Other instances of mixed colours will be soon by examining certain givy 

 substances, such as hairs, feathers, &c. with the microscope, by which it is scon grey 

 colour results from black points disseminated over a colourless or slightly-coloured 



