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PHENOMENA OF COLOURED RAYS.

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are all disposed to exhibit a red colour, and contrarily, those rays which are disposed to exhibit a red colour are all the least refrangible; so the most refrangible rays are all disposed to exhibit a deep violet, and contrarily, those which are apt to exhibit such a violet colour are all the most refrangible; and so to all the intermediate colours, in a continued series, belong intermediate degrees of refrangibility. And this analogy between colours and refrangibility is very precise and strict; the rays always agreeing exactly in both, or proportionally disagreeing in both."*

(46.) Sir Isaac Newton having, by his investigations, established the composition of white Light by analysis, proceeded to prove this by synthesis. We obtain this proof in the following manner:-If the screen upon which the spectrum is received is brought nearer the prism, the rays begin to mix; yet, even when brought close to the prism, the colours are evident. If another prism B C D, as shown by the dotted lines in the former figure, made of the same kind of glass, is placed with its angles in an opposite direction to the first prism, the coloured rays are again combined, and a white spot as before falls upon the floor.

(47.) It has been repeatedly stated that the union of the coloured rays into white Light may be illustrated by mixing together seven different powders, corresponding in colour and proportions with the prismatic rays. Such an union is said to produce a greyish white, and this falling short is attributed to the inferiority of all artificially prepared colours. The fact is, however, that no such result as that stated can be obtained. However careful we may be as to the proportions in which the powders are mixed, and in selecting those of the utmost purity, a dull neutral tint approaching to black, and not to white, will be ob

*Newton's Optics.

tained. The rotation of a disc coloured to correspond with the spectral rays, does give rise to a grey white. Not because the rays from those surfaces blend into white, but because they pass the eye so rapidly, that no one of them has time to fix its impression on the retina before it is obliterated by the next in order.

(48.) Sir David Brewster instituted an examination of the compound nature of Light by absorption; that is by viewing the spectrum after the rays have been subjected to the absorptive action of different colour media. That my readers may be in a position to value the reasonings of Sir David Brewster, I shall rapidly analyse his experiments and observations.

1. Simple inspection of the spectrum proves that red Light exists in the red, orange, and violet rays; that is, that according to the measurements of Frannhofer 192 parts out of the 360 give direct evidence of red Light.

2. The eye detects yellow Light in the orange, yellow, and green, and these rays occupy 100 parts of the spectrum.

3. Blue Light is seen to exist in the violet, indigo, blue and green spaces, therefore occupying 250 parts out of the 360.

(49.) (a.) If the blue and indigo rays are examined through a certain thickness of olive oil, they acquire a perceptible violet tint; proving the existence of red in the blue and indigo, as well as in the violet rays.

(b.) Seen through a certain thickness of balsam of sulphur, the red rays show a green band; these are considered necessarily, therefore, as containing blue and yellow.

(c.) White Light in all parts of the spectrum may be proved: when the spectrum is viewed through a considerable thickness of blue glass it displays only a yellow colour, and if the thickness is increased it becomes a greenish white. A solution of sulphate of copper and red ink produces the same effect.

Proceeding in this manner we are enabled to trace red,

ANALYSIS OF WHITE LIGHT.

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yellow, blue, and also white Light over every part of the spectrum.

(50.) "Difference of colour," says this philosopher, "is therefore not a test of difference of refrangibility, and the conclusion deduced by Newton is no longer admissible as a general truth." For a full examination of this question I must refer to the original papers. * The following are this author's general conclusions:

1. Red, yellow, and blue exist at every point of the solar spectrum.

2. As certain portions of red, yellow, and blue constitute white Light, the colour of every point of the spectrum may be considered as consisting of the predominating colour at any point mixed with white Light. In the red spaces there is more red than is necessary to make white Light with the small portions of yellow and blue which exist there; in the yellow space there is more yellow than is necessary to make white Light with the red and blue; and in that part of the blue space which appears violet there is more red than yellow; and hence, the excess of red forms a violet with the blue.

3. By absorbing the excess of any colour at any point of the spectrum above what is necessary to form white Light, we may actually cause white Light to appear at that point, and this white Light will possess the remarkable property of remaining white after any number of refractions, and of being decomposable only by absorption.

(51.) Slight consideration will show that these views suppose these coloured bands to be composed of three primary spectra of equal lengths,-a red, a yellow, and a blue spectrum overlapping each other, and exhibiting the colours of the Newtonian spectrum by allowing one to be seen through that one which overlaps it.

(52.) M. Bernard of Bordeaux has shown,-1st, That

Transactions of the Royal Society of Edinburgh, vol. xii.; and "A Treatise on Optics," by Sir David Brewster. Edin. 1853.

the intensity of the Light has such influence on the sensation of colour, that it may not only modify the aspect of the entire spectrum, but certain tints may disappear altogether. 2nd, That the absorption produced by the action of media hitherto employed on the tints of the spectrum, only affects the intensity of the light, and does not influence the nature of the colours; and 3rd, That far from destroying the bond which appears to exist between refrangibility and colouration, observations made with care tend to confirm the opposite opinion; every thing, indeed, leads to the belief that to each ray of a given refrangibility, and possessing a determined intensity, corresponds a colour susceptible of being reproduced, identically under like

circumstances.

(53.) M. Helmholtz has recently subjected the spectrum to a searching analysis; and the result is opposed to the views of Brewster, while they confirm those of Newton.* M. Helmholtz is disposed to refer the phenomena observed by Brewster, when viewing the spectrum through differently coloured media, to a diffusion of the light of the adjoining rays, over the particular ray under examination ; and he supposes this to arise from extra refraction in the prism and in the transparent coloured lamina employed, by dust, striæ, and the like producing secondary images. Helmholtz has adopted the following arrangement, to make the experiment in such a manner as to avoid all influence of diffusion. A solar spectrum is produced in the usual way, by means of a prism, and a lens placed at a suitable distance from a narrow slit admitting the solar rays. The screen which receives the spectrum is itself perforated by a slit, which can be adjusted at will to any colour; in this way is insulated a very slender luminous pencil of any of

*On the Theory of Compound Colours. Poggendorff's Annales, 1852. Phil. Mag. 1854, vol. iv.

On Sir David Brewster's New Analysis of Light. Ibid. Phil. Mag.

ibid.

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the rays under examination, which are rendered thus perfectly homogeneous. This pencil is received on a second prism, to which succeeds a lens: the group of homogeneous rays throws upon a suitably adjusted screen a narrow image of the slip. It will be evident that by such an arrangement as this, a pencil of light may be obtained which will be pure, the very trifling quantity of diffused light by which it may be accompanied being too feeble to be taken into account. The results obtained by this method support the Newtonian law of the strict relation of colour to the refracting angle. For example, pure yellow seen through blue glass of any thickness whatsoever, always preserves its yellow tint, never passing into

white.

(54.) Such is the state of the discussion as to the constitution of the spectrum. Whether the theory of the seven prismatic rays of Newton are to be adopted, or the three superposed spectra of Brewster, it is evident it must undergo much modification to meet the requirements of our more advanced knowledge. Sir John Herschel has shown us, that by looking at the spectrum with a cobalt-blue glass, we perceive a ray, called by him the "extreme red," of a crimson colour, below the ordinary red ray of the spectrum. Again, by throwing the spectrum upon paper stained yellow by turmeric, a ray of high refrangibility beyond the violet becomes visible, which ray is of a peculiar neutral colour, and has been termed a grey or lavender ray. Thus the researches of Herschel increase the number of chromatic bands to nine.

(55.) The peculiarity exhibited by a solution of sulphate of quinine in water, acidulated with sulphuric acid, had often excited attention. The solution is transparent and colourless, when we look through it; but when looking

See Article Light, Encyclopedia Metropolitana, and On the Chemical Action of the Rays of the Solar Spectrum. Philosophical Transactions, Part I. for 1840.

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