When the sun's light is first through one aperture, and then through another at some distance from the first, and is afterwards made to fall on a prism, as the rays come only from a part of the sun's disk, the spectrum has nearly the same length as before, but the breadth is greatly diminished ; in consequence of which, the light at each point is purer, it is free from penumbra, and the confines of the different colours can be more accurately traced. It was in this way that Newton measured the extent of each colour, and taking the mean of a great number of measures, he assigned the following proportions, dividing the whole length of the spectrum, exclusive of its rounded terminations, into 360 equal parts; of these the Between the divisions of the spectrum, thus made by the different colours, and the divisions of the monochord by the notes of music, Newton conceived that there was an analogy, and indeed an identity of ratios; but experience has since shown that this analogy was accidental, as the species occupied by the different colours do not di vide the spectrum in the same ratio, when prisms of different kinds of glass are employed.
Such were the experiments by which Newton first " untwisted all the shining robe of day," and made known the texture of the magic garment which nature has so kind ly spread over the surface of the visible world. From them it followed, that colours are not qualities which light derives from refraction or reflection, but are original and connate properties connected with the different degrees of refrangibility that belong to the different rays. The same colour is always joined to the same degree of refrangibi lity, and conversely, the same degree of refrangibility to the same colour.
Though the seven already enumerated are primary and simple colours, any of them may also be produced by a mixture of others. A mixture of yellow and blue, for instance, makes green ; of red and yellow orange; and, in general, if two colours, which are not very far asunder in the natural series, be mixed together, they com pound the colour that is in the middle between them.
But the most surprising composition of all, Newton observes, is that of whiteness ; which is not produced by one sort of rays, but by the mixture of all the colours in a certain proportion, namely, in that proportion which they have in the solar spectrum. This fact may be said to be made out both by analysis and composition. The white light of the sun can be separated, as we have just seen, into the seven simple colours ; and if these colours be united again they form white. Should any of them have been wanting, or not in its due proportion, the white produced is defective.
It appeared, too, that natural bodies, of whatever colour, if viewed by simple and homogeneous light, are seen of the colour of that light and of no other. Newton tried
this very satisfactory experiment on bodies of all colours, and found it to hold uniform ly ; the light was never changed by the colour of the body that reflected it.
Newton, Thus furnished with so many new and accurate notions concerning the na ture and production of colour, proceeded to apply them to the explanation of pheno mena. The subject which naturally offered itself the first to this analysis was the rain bow, which, by the grandeur and simplicity of its figure, added to tile brilliancy of its colours, in every age has equally attracted the attention of the peasant and of the phi losopher. That two refractions and one reflection were at least a part of the machi nery which nature employed in the construction of this splendid arch, had been known from the time of Antonio de Dominis and the manner in which the arched figure is produced had been shown by Descartes; so that it only remained to explain the na ture of the colour and its distribution. As the colours were the same with those ex hibited by the prism, and succeeded in the same order, it could hardly be doubted that the cause was the same. Newton showed the truth of his principles by calculating the extent of the arch, the breadth of the coloured bow, the position of the secondary bow, its distance from the primary, and by explaining the inversion of the colours' There is not, perhaps, in science any happier application of theory, or any in which the mind rests with fuller confidence.
Other meteoric appearances seemed to be capable of similar explanations, but the phenomena being no where so regular or so readily subjected to measurement as those of the rainbow, the theory cannot be brought to so severe a test, nor the evidence refl.. dered so satisfactory.
But a more difficult task remained,—to explain the permanent colour of natural bodies. Here, however, as it cannot be doubted that all colour comes from the rays of light, so we must conclude that one body is red and another violet, because the one is disposed to reflect the red or least refrangible rays, and the other to reflect the violet or the most refrangible. Every body manifests its disposition to reflect the light of its own peculiar colour, by this,• that if you cast on it pure light, first of its own colour, and then of any other, it will reflect the first much more copiously than the second. If cinnabar, for example, and ultra-marine blue be both exposed to the same red ho mogeneous light, they will both appear red ; but the cinnabar strongly luminous and resplendent, and the ultra-marine of a faint obscure red. If the homogeneal light thrown on them be blue, the converse of the above will take place.