A new party to the discussion appeared in the field in the person of M. Klingenstierna, a Swedish astronomer, who advanced some mathematical reasoning against the law of Newton, and some suspicions as to the correctness of his experiment. The latter being thus formally attacked, Mr. Dollond determined to repeat it, with a view of settling the question, and his result was communicated to the Royal Society in 1758. By placing a prism of flint glass inside one of water, confined by glass planes, so that the refractions from the two prisms should be in contrary directions, he found that when their angles were so adjusted, that the refraction of one should entirely destroy that of the other, the colour was far from being destroyed; “for the object, though not at all refracted, was yet as much infested with prismatic colours, as if it had been seen through a glass wedge only, whose refracting angle was near thirty degrees.” It was thus proved that the correction of refraction, and the correction of dispersion, are not necessarily consequent the one on the other. Previously to communicating this result, Dollond had, in 1757, applied it to the construction of achromatic glasses, consisting of spherical lenses with water between them: but finding that the images, though free from colour, were not very distinct, he tried combinations of different kinds of glass; and succeeded at last in forming the achromatic object-glass now used, consisting of a convex lens of crown, and a concave of flint glass. His son afterwards, in 1765, constructed the triple object-glass, having a double concave lens of flint glass in the middle of two double convex lenses of crown glass. The right of Dollond to the invention has been attacked by various foreign writers, but the point seems to have been decided in his favour by the general consent of later times. His conduct certainly appears more philosophical than that of either of his opponents. So long as he believed that Newton’s experiment was correct, he held fast by it, not allowing any mathematical reasoning to shake his belief, and in this respect he was more consistent than Euler, who seems to have thought that an achromatic combination might be made out of the joint belief of an experiment, and of an hypothesis utterly at variance with it. And the manner in which the distinguished philosopher just mentioned received the news of Dollond’s invention, appears singular, considering the side which each had taken in the previous discussion. Euler, who had asserted the possibility of an achromatic lens, against Dollond, who appeared to doubt it, says, “I am not ashamed frankly to avow that the first accounts which were published of it, appeared so suspicious, and even so contrary to the best established principles, that I could not prevail upon myself to give credit to them.” Dollond was the first who actually resorted to experiment, and he thus became the discoverer of a remarkable law of optics; while his tact in the application of his principles, and the selection of his materials, is worthy of admiration. The reputation of Dollond rests upon the discovery of the law, and its application to the case in point; for it has since been proved that he was not absolutely the first who had constructed an achromatic lens. On the occasion of an action brought for the invasion of the patent, the defendant proved that about the year 1750, Dr. Hall, an Essex gentleman, was in the possession of a secret for constructing achromatic telescopes of twenty inches focal length: and a writer in the Gentleman’s Magazine for 1790, has advanced his claim with considerable circumstantial detail. It is difficult to get any account of that trial, as it is not reported in any of the books. At least we presume so, from not finding any reference to it either in the works of Godson or Davis on Patents, though the case is frequently mentioned; or in H. Blackstone’s report of Boulton and Watt v. Bull, in which Dollond’s case forms a prominent feature of the argument. But, from the words of Judge Buller in the case just cited, it is difficult to suppose that the account given by Lalande (Montucla, Histoire des Mathématiques, vol. iii. p. 448, note) can be correct. Lalande asserts that it was proved that Dollond received the invention from a workman who bad been employed by Dr. Hall, and that the latter had shown it to many persons. Judge Buller says, “The objection to Dollond’s patent was, that he was not the inventor of the new method of making object-glasses, but that Dr. Hall had made the same discovery before him. But it was holden that as Dr.