|Hutton, Charles Mathematical and Philosophical Dictionary 1795|
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,