Another combatant soon sprung up in the person of one Francis Linus, a physician in Liege,[13] who, on the 6th October, 1674, addressed a letter to a friend in London, containing animadversions on Newton’s doctrine of colours. He boldly affirms, that in a perfectly clear sky the image of the sun made by a prism is never elongated, and that the spectrum observed by Newton was not formed by the true sunbeams, but by rays proceeding from some bright cloud. In support of these assertions, he appeals to frequently repeated experiments on the refractions and reflections of light which he had exhibited thirty years before to Sir Kenelm Digby, “who took notes upon them;” and he unblushingly states, that, if Newton had used the same industry as he did, he would never have “taken so impossible a task in hand, as to explain the difference between the length and breadth of the spectrum by the received laws of refraction.” When this letter was shown to Newton, he refused to answer it; but a letter was sent to Linus referring him to the answer to Pardies, and assuring him that the experiments on the spectrum were made when there was no bright cloud in the heavens. This reply, however, did not satisfy the Dutch experimentalist. On the 25th February, 1675, he addressed another letter to his friend, in which he gravely attempts to prove that the experiment of Newton was not made in a clear day;—that the prism was not close to the hole,—and that the length of the spectrum was not perpendicular, or parallel to the length of the prism. Such assertions could not but irritate even the patient mind of Newton. He more than once declined the earnest request of Oldenburg to answer these observations; he stated, that, as the dispute referred to matters of fact, it could only be decided before competent witnesses, and he referred to the testimony of those who had seen his experiments. The entreaties of Oldenburg, however, prevailed over his own better judgment, and, “lest Mr. Linus should make the more stir,” this great man was compelled to draw up a long and explanatory reply to reasonings utterly contemptible, and to assertions altogether unfounded. This answer, dated November 13th, 1675, could scarcely have been perused by Linus, who was dead on the 15th December, when his pupil Mr. Gascoigne, took up the gauntlet, and declared that Linus had shown to various persons in Liege the experiment which proved the spectrum to be circular, and that Sir Isaac could not be more confident on his side than they were on the other. He admitted, however, that the different results might arise from different ways of placing the prism. Pleased with the “handsome genius of Mr. Gascoigne’s letter,” Newton replied even to it, and suggested that the spectrum seen by Linus may have been the circular one, formed by one reflexion, or, what he thought more probable, the circular one formed by two refractions, and one intervening reflection