"This experiment is the very same with that of Sir Isaac Newton above mentioned, notwithstanding the result was so remarkably different: but Mr Dollond affirms us, that he used all possible precaution and care in his process; and he kept his apparatus by him, that he might evince the truth of what he wrote, whenever he should be properly required to do it.

"He plainly saw, however, that if the refracting angle of the water vessel 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 inconvenience 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°, made it necessary to try some experiments of the same kind with smaller angles.

"Accordingly he got a wedge of plate-glass, the angle of which was only nine 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.

"Notwithstanding it evidently appeared, I may say to almost all philosophers, that Mr Dollond had made Sir Isaac a real discovery of something not comprehended in the optical principles of Sir Isaac Newton, it did not appear to be sensible a man, and so good a mathematician as Mr Murdoch is universally acknowledged to be. Upon this occasion he interposed in the defence, as he imagined, of Sir Isaac Newton; maintaining, that Mr Dollond's positions, which he says, he knows not by what mischance have been deemed paradoxes in... Sir Isaac's theory of light, are really the necessary consequences of it. He also endeavours to show, that Sir Isaac might not be mistaken in his account of the experiment above mentioned. But admitting all that he advances in this part of his defence, Newton must have made use of a prism with a much smaller refracting angle than, from his own account of his experiments, we have any reason to believe he ever did make use of.

"The fact probably was, that Sir Isaac deceived himself in this case, by attending to what he imagined to be the clear consequences of his other experiments; and though the light he saw was certainly tinged with colours, and he must have seen it to be so, yet he might imagine that this circumstance arose from some imperfection in his prisms, or in the disposition of them, which he did not think it worth his while to examine. It is also observable, that Sir Isaac is not so particular in his description of his prisms, and other parts of his apparatus, in his account of this experiment, as he generally is in other cases, and therefore probably wrote his account of it from his memory only.

Much has been said on this experiment; and it is thought very extraordinary, that a man of Sir Isaac's accurate attention should have overlooked a circumstance, the effect of which now appears to be so considerable. But it has happily occurred to Mr Mitchel, that, as Sir Isaac Newton observes, he used to put saccharum saturni into his water to increase its refractive power, the lead, even in this form, might increase the dissipative refraction, as it does in the composition of glass; and if so, that this would account for Newton's not finding his dissipative power of water less than that of the glass prisms, which he otherwise ought to have done, if he had tried the experiment as he said he did.

Accordingly he included a prism of glass in water, as highly impregnated with saccharum saturni as it would bear, the proportion of saccharum to water being about as 5 to 11. When the image, seen through the water (so impregnated) and a glass prism, was in its natural place, it still was coloured, though very little: he thought not more than a fourth part as much as when seen through the plain water, and the prism in its natural place; so that he had no doubt, but that, if his prism had had a little less of the dispersing power, its errors would have been perfectly corrected."

Besides the experiments of Mr Delaval above related, and which were made 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. These arts, he observes, were in very remote ages carried to the utmost height of perfection in the countries of Phoenicia, Egypt, Palestine, India, &c., and that the inhabitants of these countries also excelled in the art of imitating gems, and tinging glass and enamel of various colours. The colours used in very ancient paintings were as various as those now in use, and greatly superior both in beauty and durability. The paints used by Apelles were so bright, that he was obliged to glaze his pictures with a dark coloured varnish, lest the eye should be offended by their excessive brightness; and even these were inferior to what had been used among the ancient Egyptians. Pliny complains that the art of painting was greatly decayed in his time; and the moderns were not furnished with any means of retrieving the art, until they began to avail themselves of experimental observations.

The changes of colour in permanently coloured bodies, our author observes, are produced by the famous delays which take place in transparent colourless substances; and the experiments by which they can be divination investigated consist chiefly of various methods of uniting the colouring particles into larger, or dividing them into smaller masses. Sir Isaac Newton made his experiments chiefly on transparent substances; and in the few places where he treats of others, acknowledges his deficiency 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." It was the opinion of this great philosopher, that all coloured matter reflects the rays of light, some reflecting the more refrangible, and others the less refrangible rays more copiously; and that this is not only a true reason of these colours, but likewise the only reason. He was likewise of opinion, that opaque bodies reflect the light from their interior surface by some power of the body evenly diffused over and external to it. With regard to transparent-coloured liquors, he expresses himself in the following manner: "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 tingling particles may predominate. In such cases, the colour of the reflected light will be apt to vary from that of the light transmitted."

To investigate 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 semi-transparent bodies. From these 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 (A).

This incapacity of the colouring particles of trans-

(A) Here our author observes, that he makes use of the word colour only to express those called primary; such parent bodies to reflect light, being deduced from very numerous experiments, may therefore be held as a general law. It will appear the more extensive, if we consider, that, for the most part, the tinged 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.

For making his experiments, Mr Delaval used small phials of flint-glass, whose form was a parallelopiped, and their height, exclusive of the neck, about two inches, the base about an inch square, and the neck two inches in length. The bottom and three sides of each of these phials were covered with a black varnish; the cylindrical neck, and the anterior side, except at its edges, being left uncovered. He was careful to avoid any crevices in the varnish, that no light might be admitted except through the neck or anterior side of the phials.