82 



COLOUR. 



objects become quite invisible at a certain depth 

 below the surface of water. So also on the top of 

 the Andes or Alps, many more stars are visible to 

 the naked eye, than in the vallej's or plains below ; 

 because their rays having to pass through a smaller 

 xtent of air, less of the light which they impart 

 becomes absorbed. These facts are recognised, and 

 different explanations of them proposed by the sup- 

 porters both of the molecular and undulatory theory 

 of light, and require to be remembered in studying 

 the colours which different natural bodies assume. 



It was known to the ancients, that the light of the 

 SUB might be so acted upon by different objects, as to 

 exhibit a variety of colours ; thus Aristotle ascribed 

 the appearance of the rainbow to the reflection of the 

 sun's rays on the falling drops of rain ; and Seneca 

 states, that if the sun shine through a triangular piece 

 of glass, the colours of the rainbow will be displayed. 

 Indeed the exhibition of colours by the prism had 

 often been made previous to the time of Sir Isaac 

 Newton; it remained, however, for that illustrious 

 philosopher to examine the phenomena more criti- 

 cally, and deduce from his analysis, more definite and 

 important inferences than had been previously at- 

 tempted. His method of proceeding was simple, 

 and may be readily understood. In the first place 

 he darkened a room, admitting through a hole in the 

 window-shutter only a small quantity of light, which 

 was reflected upon the opposite wall. He then, 

 taking the triangular piece of glass or prism, inter- 

 posed it in the course of the sunbeam, and observed, 

 that instead of being simply directed out of its course, 

 and forming the same circular image which it had 

 previously done upon the wall, an oblong image of 

 the sun appeared, presenting seven different colours. 

 This surprised him, and he states that it " excited in 

 him a more than ordinary curiosity to examine from 

 whence it might proceed ;" accordingly, apprehensive 

 that some unevenness in the glass or other incidental 

 circumstance might have occasioned a deception, he 

 repeated the experiment in a variety of ways, yet 

 always with a similar result. He then began to think, 

 that what appeared to be white light must in reality 

 be composed of these seven different colours; to 

 determine this more satisfactorily, he took another 

 prism, and so adjusted it as to refract the light in a 

 contrary direction, so that the primary or different 

 coloured rays were again blended together, upon 

 which the white light was reproduced. Hence he 

 inferred, that what seems to be homogeneous white 

 light, is a combination of seven different coloured 

 rays, red, orange, yellow, green, blue, indigo, and 

 violet; and he then proceeded to examine these 

 different colours separately. In so doing, he allowed 

 each particular colour to fall on a second prism, and 

 observed that when refracted it was not, as before, 

 drawn into an oblong image, nor into any other 

 colour, wherefore he concluded that each of the seven 

 primary rays of light possessed a specific degree of 

 refrangibility, and that there is an immutable corre- 

 spondence between the refrangibility of each ray, and 

 the colour it possesses. This comprehends what is 

 meant by the different refrangibilities of the rays of 

 light ; for it was observed that the red was less re- 

 frangible than the orange; the orange than the 

 yellow ; the yellow than the green ; the blue than 

 the indigo ; and that the violet was the most refran- 

 gible of them all. The different colours so exhibited 

 form what is described as the solar spectrum ; but it 



is to be observed, that by Sir Isaac Newton's analysis 

 no lines or boundaries between the different colours 

 were perceptible ; the colours appearing to be shaded 

 off by imperceptible gradations into one another, 

 and he determined their relative proportions by the 

 individual length or spaces they occupied. It has 

 since been ascertained by Dr. Wollaston, aided by 

 Fraunhofer, that when the prism is perfect, and the 

 light admitted through a very small aperture, the 

 spectrum received on a sheet of white paper will 

 exhibit the prismatic colours, divided by an indefinite 

 number of dark and sometimes black lines, which in 

 all spectra formed by solar light, preserve the same 

 intensity and relative positions, so that they may be 

 esteemed fixed points for ascertaining the refractive 

 powers of different media. It is worthy however of 

 observation, that they differ with the species of light 

 employed, the spectra of the moon and brighter fixed 

 stars, exhibiting lines peculiar to themselves. Further- 

 more, it appears that Sir Isaac Newton erred, in not 

 observing that the relations of the coloured spaces 

 must be considerably modified by the angular magni- 

 tude of the sun, or luminous body, or the aperture 

 from which his spectrum was obtained; hence his 

 proposition, " that to the same degree of refrangibility 

 ever belongs the same colour, and to the same colour 

 ever belongs the same refrangibility," proves to have 

 been incorrect. The researches of Sir David Brew- 

 ster have indeed effected a still further analysis of 

 the solar spectrum. Taking advantage of the property 

 which certain coloured media have, of absorbing one 

 or more of the primary rays, this accomplished philo- 

 sopher has succeeded in reducing the colours of the 

 spectrum to three, viz. red, yellow, and blue; the 

 orange in the spectrum of Sir Isaac Newton, he 

 found to be occasioned by the mixture of the red 

 and yellow rays ; the green bv the mixture of the blue 

 with the yellow rays. His investigations appeal* to 

 have established the following facts concerning the 

 composition of the solar spectrum. 1. White light 

 consists of three simple colours, red, yellow, and blue, 

 by the combinations of which all other colours may 

 be formed. 2. The solar spectrum consists of three 

 spectra of equal length, beginning and terminating at 

 the same points, viz., a red spectrum, a yellow spec- 

 trum, and a blue spectrum ; all other colours in the 

 spectrum are compound colours, each consisting of 

 red, blue, and yellow light, in different proportions. 

 3. A certain quantity of white light, incapable of 

 being decomposed by the prism, in consequence of 

 all its component rays having the same refrangibility, 

 exists at every point of the spectrum, and may in 

 some points be exhibited in an isolated state. 



However the number of colours in the solar spec- 

 trum may be thus reduced, the variety of effect pro- 

 duced by their blending with each other can scarcely 

 be brought down to analytical computation. Hence 

 Professor Powell observes, " That a question often 

 asked as to the number of primary colours can only 

 be answered with reference to the sense in which it is 

 asked. If it be meant to apply to the number of 

 tints distinguished in the spectrum, this will be a 

 matter of individual judgment to different eyes. 

 Newton distinguished seven, others four, others three, 

 but perhaps most observers would admit that it is 

 impossible to fix on any number, since the light ap- 

 pears to go through every possible shade of colour 

 between the deep red and faint violet. If we under- 

 stand the question as applying to the number of 



