, is that part of optics which explains the several properties of the colours of light, and of natural bodies.

Before the time of Sir I. Newton, the notions concerning colour were very vague and wild. The Pythagoreans called colour the superficies of bodies: Plato said that it was a flame issuing from them: According to Zeno, it is the first consiguration of matter: And Aristotle said it was that which made bodies actually transparent. Descartes accounted colour a modification of light, and he imagined that the difference of colour proceeds from the prevalence of the direct or rotatory motion of the particles of light. Grimaldi, Dechales, and many others, imagined that the differences of colour depended upon the quick or slow vibrations of a certain elastic medium with which the universe is silled. Rohault conceived, that the different colours were made by the rays of light entering the eye at different angles with respect to the optic axis. And Dr. Hooke imagined that colour is caused by the sensation of the oblique or uneven pulse of light; which being capable of no more than two varieties, he concluded there could be no more than two primary colours.

Sir I. Newton, in the year 1666, began to investigate this subject; when finding that the coloured image of the sun, formed by a glass prism, was of an oblong, and not of a circular form, as, according to the laws of equal refraction, it ought to be, he conjectured that light is not homogeneal; but that it consists of rays of different colours, and endued with divers degrees of refrangibility. And, from a farther prosecution of his experiments, he concluded that the different colours of bodies arise from their reflecting this or that kind of rays most copiously. This method of accounting for the different colours of bodies soon became generally adopted, and still continues to be the most prevailing opinion. It is hence agreed that the light of the sun, which to us seems white and perfectly homogeneal, is composed of no fewer than seven different colours, viz red, orange, yellow, green, blue, purple, and violet or indigo: that a body which appears of a red colour, has the property of reflecting the red rays more plentifully than the rest; and so of the other colours, the orange, yellow, green, &c: also that a body which appears black, instead of reflecting, absorbs all or the most part of the rays that fall upon it; while, on the contrary, a body which appears white, reflects the greatest part of all the rays indiscriminately, without separating them one from another.

The foundation of a rational theory of colours being thus laid, the next inquiry was, by what peculiar mechanism, in the structure of each particular body, it was fitted to reflect one kind of rays more than another; and this is attributed, by Sir I. Newton, to the density of these bodies. Dr. Hooke had remarked, that thin transparent substances, particularly soap-water blown into bubbles, exhibited various colours, according to their thinness; and yet, when they have a considerable degree of thickness, they appear colourless. And Sir Isaac himself had observed, that as he was compressing two prisms hard together, in order to make their sides (which happened to be a little convex) to touch one another, in the place of contact they were | both perfectly transparent, as if they had been but one continued piece of glass: but round the point of contact, where the glasses were a little separated from each other, rings of different colours appeared. And when he afterwards, farther to elucidate this matter, employed two convex glasses of telescopes, pressing their convex sides upon one another, he observed several series of circles or rings of such colours, different, and of various intensities, according to their distance from the common central pellucid point of contact.

As the colours were thus found to vary according to the different distances between the glass plates, Sir Isaac conceived that they proceeded from the different thickness of the plate of air intercepted between the glasses; this plate of air being, by the mere circumstance of thinness or thickness, disposed to reflect or transmit the rays of this or that particular colour. Hence therefore he concluded, that the colours of all natural bodies depend on their density, or the magnitude of their component particles: and hence also he constructed a table, in which the thickness of a plate necessary to reflect any particular colour, was expressed in millionth parts of an inch.

From a great variety of such experiments, and observations upon them, our author deduced his theory of colours. And hence it seems that every substance in nature is transparent, provided it be made sufficiently thin; as gold, the densest substance we know of, when reduced into thin leaves, transmits a bluish-green light through it. If we suppose any body therefore, as gold for instance, to be divided into a vast number of plates, so thin as to be almost perfectly transparent; it is evident that all, or the greatest part of the rays, will pass through the upper plates, and when they lose their force will be reflected from the under ones. They will then have the same number of plates to pass through which they had penetrated before; and thus, according to the number of those plates through which they are obliged to pass, the object appears of this or that colour, just as the rings of colours appeared different in the experiment of the two plates, according to their distance from one another, or the thickness of the plate of air between them.

This theory of the colours has been illustrated and confirmed by various experiments, made by other phylosophers. Mr. E. H. Delaval produced similar effects by the infusions of slowers of different colours, and by the intimate mixture of the metals with the substance of glass, when they are reduced to very fine parts; the more dense metals imparting to the glass the less refrangible colours, and the lighter ones those colours that are more easily refrangible. Dr. Priestley and Mr. Canton also, by laying very thin leaves or slips of the metals upon glass, ivory, wood, or metal, and passing an electrical stroke through them, found that the same effect was produced, viz, that the substrated was tinged with different colours, according to the distance from the point of explosion.

However, the Abbe Mazeas and M. du Tour contended, that the colours between the glasses are not to be ascribed to the thin stratum of air, since they equally produced them by rubbing and pressing together two flat glasses, which cohered so closely that it required the greatest force to move or slide them over one another. See Priestley's History of Vision.

Of Newton's 8th Exper. in the 2d Book of Optics.

The event of this experiment, which has been contradicted by repetitions of the same by other philosophers, having been the occasion of much controversy; and relating to a material part of the doctrine of chromatics, it will not be improper here to give an account of what has passed concerning it. Newton found, he says, that when light, by contrary refractions through different mediums, is so corrected, that it emerges in lines parallel to the incident rays, it continues ever after to be white. But that if the emergent rays be inclined to the incident ones, the whiteness of the emerging light will, by degrees, in passing on from the place of emergence, become tinged at its edges with colours. And these laws he inferred from experiments made by refracting light with prisms of glass, placed within a prismatic vessel of water.

By theorems deduced from this experiment he infers, that the refraction of the rays of every sort, made out of any medium into air, are known by having the refraction of the rays of any one sort: and also, that the refraction out of one medium into another is found, whenever we have the refractions out of them both, into any third medium.

Now the same experiment, when since performed by other persons, turning out contrary to what is stated above, some rather free reflections have been thrown upon Newton concerning it; but which however have been very satisfactorily obviated by Mr. Peter Dollond, in a late pamphlet on this subject; as we shall shew below.

In the first place then, M. Klingenstierna, a Swedish philosopher, having in the year 1755 considered the controversy between Euler and Mr. John Dollond, relative to the refraction of light, formed a theorem of his own, from geometrical reasoning, by which he was induced to believe that the result of Newton's experiment could not be as he had related it; except when the angles of the refracting mediums are small. See the paper on this matter by Klingenstierna in the pamphlet above cited by Mr. Peter Dollond.

This paper of Klingenstierna being communicated to Mr. John Dollond by Mr. Mallet, to whom it was sent for that purpose, made Dollond entertain doubts concerning Newton's report of the result of his experiment, and determined him to have recourse to experiments of his own, which he did in the year 1757, as follows.

He cemented two glass planes together by their edges, so as to form a prismatic vessel when closed at the ends or bases; and the edge being turned downward, he placed it in a glass prism with one of its edges upward, filling up the vacancy with clear water; so that the refraction of the prism was contrived to be contrary to that of the water, in order that a ray of light, transmitted through both these refracting mediums, might be affected by the difference only between the two refractions. As he found the water to refract more or less than the glass prism, he diminished or augmented the angle between the glass plates, till the two contrary refractions became equal, which he discovered by view- | ing an object through this double prism. For when it appeared neither raised nor depressed, he was satisfied that the refractions were equal, and that the emergent rays were parallel to the incident ones.

Now, according to the prevailing opinion, he observes, that the object ought to have appeared through this double prism in its natural colour; for if the difference of refrangibility had been in all respects equal, in the two equal refractions, they would have rectified each other. But this experiment fully proved the fallacy of the received opinion, by shewing that the divergency of the light by the glass prism, was almost double of that by the water; for the image of the object, though not at all refracted, was yet as much infected with prismatic colours, as if it had been seen through a glass wedge only, having its angle of near 30 degrees.

This experiment is the very same with that of Sir Isaac Newton above-mentioned, not withstanding the result was so remarkably different. Mr. Dollond plainly saw however, that if the refracting angle of the watervessel could have admitted of a sufficient increase, the divergency of the coloured rays would have been greatly diminished, or entirely rectified; and that there would have been a very great refraction without colour, as he had already produced a great discolouring without refraction: but the inconveniency of so large an angle as that of the prismatic vessel must have been, to bring the light to an equal divergency with that of the glass prism, whose angle was about 60 degrees, made it necessary to try some experiments of the same kind with smaller angles.

Accordingly he procured a wedge of plate-glass, whose angle was only 9 degrees; and, using it in the same circumstances, he increased the angle of the water-wedge, in which it was placed, till the divergency of the light by the water was equal to that by the glass; that is, till the image of the object, though considerably refracted by the excess of the refraction of the water, appeared nevertheless quite free from any colours proceeding from the different refrangibility of the light.

Many conjectures were made as to the cause of so striking a difference in the results of the same experiment; but none that gave any great satisfaction, till lately that it has been shewn to be probably owing to the nature of the glass then used by Newton. This conjecture is made by Mr. Peter Dollond, son of John, the inventor of the achromatic telescope, in a pamphlet by him lately published in defence of his father's invention, against the misrepresentations of some persons who have unjustly attempted to give the invention to other philosophers, who themselves never imagined that they had any right to it. After a full and satisfactory vindication of his father, Mr. P. Dollond then adds,

“I now come to a more agreeable part of this paper, which is, to endeavour to reconcile the different results of the 8th experiment of the 2d part of the 1st book of Newton's Optics, as related by himself, and as it was found by Dollond, when he tried the same experiment, in the year 1757. Newton says, that light, as often as by contrary refractions it is so corrected, that it emergeth in lines parallel to the incident, continues ever after to be white. Now Dollond says, when he tried the same experiment, and made the mean refraction of the water equal to that of the glass prism, so that the light emerged in lines parallel to the incident, he found the divergency of the light by the glass prism to be nearly double to what it was by the water prism. The light appeared to be so evidently coloured, that it was directly said by some persons, that if Newton had actually tried the experiment, he must have perceived it to have been so. Yet who could for a moment doubt the veracity of such a character? Therefore different conjectures were made by different persons. Mr. Murdoch in particular gave a paper to the Royal Society in defence of Newton; but it was such as very little tended to clear up the matter. Philos. Trans. vol. 53. pa. 192.—Some have supposed that Newton made use of water strongly impregnated with saccharum saturni, because he mentions sometimes using such water, to increase the refraction, when he used water prisms instead of glass prisms. Newton's Opt. pa. 62.—And others have supposed, that he tried the experiment with so strong a persuasion in his own mind that the divergency of the colours was always in the same proportion to the mean refraction, in all sorts of refracting mediums, that he did not attend so much to that experiment as he ought to have done, or as he usually did. None of these suppositions having appeared at all satissactory, I have therefore endeavoured to find out the true cause of the difference, and thereby shew, how the experiment may be made to agree with Newton's description of it, and to get rid of those doubts, which have hitherto remained to be cleared up.

“It is well known, that in Newton's time the English were not the most famous for making optical instruments: Telescopes, opera-glasses, &c, were imported from Italy in great numbers, and particularly from Venice; where they manufactured a kind of glass which was much more proper for optical purposes than any made in England at that time. The glass made at Venice was nearly of the same refractive quality as our own crown-glass, but of a much better colour, being sufficiently clear and transparent for the purpose of prisms. It is probable that Newton's prisms were made with this kind of glass; and it appears to be the more so, because he mentions the specific gravity of common glass to be to water as 2.58 to 1, Newton's Opt. pa. 247, which nearly answers to the specisic gravity we sind the Venetian glass generally to have. Having a very thick plate of this kind of glass, which was presented to me about 25 years ago by the late professor Allemand, of Leyden, and which he then informed me had been made many years; I cut a piece from this plate of glass to form a prism, which I conceived would be similar to those made use of by Newton himself. I have tried the Newtonian experiment with this prism, and find it answers so nearly to what Newton relates, that the difference which remains may very easily be supposed to arise from any little difference which may and does often happen in the same kind of glass made at the same place at different times. Now the glass prism made use of by Dollond to try the same experiment, was made of English flint-glass, the specific gravity of which I have never known to be less than 3.22. This difference in the densities of the prisms, | used by Newton and Dollond, was sufficient to cause all the difference which appeared to the two experimenters in trying the same experiment.

“From this it appears, that Newton was accurate in this experiment as in all others, and that his not having discovered that, which was discovered by Dollond so many years afterwards, was owing entirely to accident; for if his prism had been made of glass of a greater or less density, he would certainly have then made the discovery, and refracting telescopes would not have remained so long in their original imperfect state.” See Achromatic, and Telescope.

Mr. Delaval's experiments on the colours of opaque bodies.—Beside the experiments of this gentleman, before-mentioned, on the colours of transparent bodies, he has lately published an account of some made upon the permanent colours of opaque substances, the discovery of which must be of the utmost consequence in the arts of colour-making and dyeing.

The changes of colour in permanently coloured bodies, our author observes, are produced by the same laws that take place in transparent colourless substances; and the experiments by which they are investigated consist chiefly of various methods of uniting the colouring particles into larger masses, or dividing them into smaller ones. Sir Isaac Newton made his experiments chiefly on transparent substances; and in the few places where he treats of others, he acknowledges his want of experiments. He makes the following remark however on those bodies which reflect one kind of light and transmit another, viz, that if these glasses or liquors were so thick and massy that no light could get through them, he questioned whether they would not, like other opaque bodies, appear of one and the same colour in all positions of the eye; though he could not yet affirm it from experience. Indeed it was the opinion of this great philosopher, that all coloured matter reflects the rays of light, some reflecting the more refrangible rays most copiously, and others those that are less so; and that this is at once the true and only reason of these colours. He was likewise of opinion that opaque bodies reflect the light from their anterior surface, by some power of the body evenly disfused over and external to it. With respect to transparent coloured bodies he thus expresses himself: “A transparent body which looks of any colour by transmitted light, may also look of the same colour by reflected light; the light of that colour being reflected by the farther surface of that body, or by the air beyond it: and then the reflected colour will be diminished, and perhaps cease, by making the body very thick, and pitching it on the back-side to diminish the reflection of its farther surface, so that the light reflected from the tinging particles may predominate. In such cases the colour of the reflected light will be apt to vary from that of the light transmitted.”

To search out the truth of these opinions, Mr. Delaval entered upon a course of experiments with transparent coloured liquors and glasses, as well as with opaque and semitransparent bodies. And from these experiments he discovered several remarkable properties of the colouring matter; particularly, that in transparent coloured substances it does not reflect any light; and when, by intercepting the light which was transmitted, it is hindered from passing, through such substances, they do not vary from their former colour to any other, but become entirely black.

This incapacity of the colouring particles of transparent bodies to reflect light, being deduced from very numerous experiments, may therefore be taken as a general law. It will appear the more extensive, if it be considered that, for the most part, the tinging particles of liquors, or other transparent substances, are extracted from opaque bodies; that the opaque bodies owe their colours to those particles, in like manner as the transparent substances do; and that by the loss of them they are deprived of their colours.

Notwithstanding these and many other experiments, the theory of colours seems not yet determined with certainty; and it must be acknowledged that very strong objections might be brought against every hypothesis on this subject that has been invented. The discoveries of Sir Isaac Newton however are sufficient to justify the following Aphorisms.

Aphorism 1. All the colours in nature arise from the rays of light.

2. There are seven primary colours, namely red, orange, yellow, green, blue, indigo, and violet.

3. Every ray of light may be separated into these seven primary colours.

4. The rays of light, in passing through the same medium, have different degrees of refrangibility.

5. The difference in the colours of light arises from its different refrangibility: that which is the least refrangible producing red; and that which is the most refrangible, violet.

6. By compounding any two of the primary, as red and yellow, or yellow and blue, the intermediate colour, orange or green, may be produced.

7. The colours of bodies arise from their dispositions to reflect one sort of rays, and to absorb the others: those that reflect the least refrangible rays appearing red; and those that reflect the most refrangible, violet.

8. Such bodies as reflect two or more sorts of rays, appear of various colours.

9. The whiteness of bodies arises from their disposition to reflect all the rays of light promiscuously.

10. The blackness of bodies proceeds from their incapacity to reflect any of the rays of light.—And from their thus absorbing all the rays of light that are thrown upon them, it arises, that black bodies, when exposed to the sun, become hot sooner than all others.

Of the Diatonic Scale of Colours.—Sir Isaac Newton, in the course of his investigations of the properties of light, discovered that the lengths of the spaces occupied in the spectrum by the seven primary colours, exactly correspond to the lengths of chords that sound the seven notes in the diatonic scale of music: which is made evident by the following experiment. |

On a paper, in a dark chamber, let a ray of light be largely refracted into the spectrum ABCDEF, marking upon it the precise boundaries of the several colours, as a, b, c, &c; and across the spectrum draw the perpendicular lines ag, bh, &c. Then it will be found that the spaces, by which the several colours are bounded, viz, BagF containing the red, abhg containing the orange, bcih containing the yellow, &c, will be in exact proportion to the divisions of a musical chord for the notes of an octave; that is, as the intervals of these numbers 1, 8/9, 5/6, 3/4, 2/3, 3/5, 9/16, 1/2.

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Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

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