The new theory of colours was quickly assailed by several other writers, who seem all to have had a better apology than Hooke for the errors into which they fell. Among them one of the first was Father Pardies, who wrote against the experiments, and what he was pleased to call the hypothesis, of Newton. A satisfactory and calm reply convinced him of his mistake, which he had the candour very readily to acknow ledge. A countryman of his, Mariotte, was more difficult to be reconciled, and, , though very conversant with experiment, appears never to have succeeded in re peating the experiments of Newton. Desaguliers, at the request of the latter, re peated the experiments doubted of before the Royal Society, where Monmort, a countryman and a friend of Mariotte, was present.' MM. Linus and Lucas, both of Leige, objected to Newton's experiments as inac curate ; the first, because, on attempting to repeat them, he had not obtained the same results ; and the second, because he had not been able to perceive that a red object and a blue required the focal distance to be different when they were viewed through a telescope. Newton replied with great patience and good temper to both.
The series was closed, in 1727, by the work of an Italian author, Rizetti, who, in like manner, called in question the accuracy of experiments which he himself had not been able to repeat. Newton was now no more, but Desaguliers, in consequence of Rizetti's doubts, instituted a series of experiments which seemed to set the matter en tirely at rest. These experiments are described in the Philosophical Transactions for 1728.
An inference which Newton had immediately drawn from the discoveries above described was, that the great source of imperfection in the refracting telescope was the different refrangibility of the rays of light, and that there were stronger reasons than either Mersenne or Gregory had suspected, for looking to reflection for the improve ment of optical instruments. It was evident, from the different refrangibility of light, that the rays coming from the same point of an object, when decomposed by the re fraction of a lens, must converge to different foci; the red rays, for example, to a point more distant from the lens, and the violet to one nearer by about a fifty-fourth part of the focal distance. Hence it was not merely from the aberration of the rays caused by the spherical figure of the lens that the imperfection of the images fornted by refrac tion arose, but from the very nature of refraction itself. It was evident, at the same time, that in a combination of lenses with opposite figures, one convex, for instance, and another concave, there was a tendency of the two contrary dispersions to correct one another. But it appeared to Newton, on examining different refracting substances, that the dispersion of the coloured rays never could be corrected except when the refraction itself was entirely destroyed, for he thought he bad discovered that the quan tity of the refraction and of the dispersion in different substances bore always the same proportion to one another. This is one of the few instances in which his conclusions
have not been confirmed by subsequent experiment; and it will, accordingly, fall under discussion in another part of this discourse.
Having taken the resolution of constructing a reflecting telescope, he set about doing so with his own hands. There was, indeed, at that time, no other means by which such a work could be accomplished ; the art of the ordinary glass-grinder not being sufficient to give to metallic specula the polish which was required. It was on this account that Gregory had entirely failed iu realising his very ingenious optical invention.
Newton, however, himself possessed excellent hands for mechanical operations, and could use them to better purpose than is common with men so much immersed in deep and abstract speculation. It appears, indeed, that mechanical invention was one of the powers of his mind which began to unfold itself at a very early period. In some letters subjoined to a Memoir drawn up after his death by his nephew Conduit, it is said, that, when a boy, Newton used to amuse himself with constructing machines, mills, &c. on a small scale, in which be displayed great ingenuity; and it is probable that he then acquired that use of his hands which is so difficult to be learned at a later period. To this, probably, we owe the neatness and ingenuity with which the optical experiments above referred to were contrived and executed,—experiments of so diffi cult a nature, that any error in the manipulation would easily defeat the effect, and appears actually to have done so with many of those who objected to his experi. ments.' He succeeded perfectly in the construction of his telescope, and his first communi cation with Oldenburg, and the first reference to his optical experiments,. is connect ed with the construction of this instrument, and mentioned in a letter dated the 11th January 1672. He had then been proposed as a member of the Royal Society by the Bishop of Sarum, and he says, " If the honour of being a member of the Society shall be conferred on me, I shall endeavour to testify my gratitude by communicating what my poor and solitary endeavours can effect toward the promoting its philosophical designs."' Such was the modesty of the man who was to effect a greater revolution in the state of our knowledge of nature than any individual had yet done, and greater, perhaps, than any individual is ever destined to bring about. Success, however, never altered the temper in which he began his researches.