Archimedes Project

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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-