After he had drawn up his treatise on double refrac tion, Huygens discovered what he calls a " wonderful phenomenon," or the polarization of the two pencils of light formed by Iceland spar. Hitherto he had sup posed the light to have the same properties as any other ordinary light ; but upon transmitting the two pencils formed by one piece of Iceland spar through another piece, he was surprised to observe, that when the two pieces were similarly placed, or with their principal sec tions parallel, neither of the two pencils were divided in passing through the second rhornb, the pencil which had suffered the ordinary refraction by the lirst rhomb being only ordinarily refiacted by the second, and the pencil which had been extraordinarily refracted by the first being only extraordinarily refracted by the second rhornb. Ile found also, that when the principal sections cut one another at right angles, the ray which was ordi narily refracted by the first crystal suffered only the ex traordinary refraction from the second ; and the ray which suffered the extraordinary refraction from the first suffered only the ordinary refraction from the second. But in all other positions ol the principal sec tions, excepting these two, the rays divided themselves into two, by the refraction of the second crystal, so that frorn the single pencil of light incident on the lirst crys tal, there were formed four pencils, sometimes of equal brightness, according to the different positions of the crystals, but which did not seem to have more light, when taken altogether, than the single ray. Huygens acknowledged himself unable to form any satisfaetot y conjecture respecting the nature or cause of this new property of light ; and it was reserved for Newton to show, that the sides of the rays had, in passing through the first crystal, acquired different properties.
Notwithstanding the great accuracy of Huygens's law of double refraction in Iceland spar, he never supposes that it was a general law for all other bodies. He main tains, on the contrary, that he had found by direct expe riment, that the only other doubly refracting crystal with which he was acquainted, namely, rock crystal, had its double refraction regulated by an entirely different law, the light being propagated through it in two spherical waves, one of which was a little SiONVer than the other. This result Ivas obtained by cutting prisms of rock crys tal in different directions, and is mentioned in Itis pre face as a new remark, which he had made after the first copy of his treatise had been communicated to some of the members of the Academy of Sciences. Iluygens, therefore, proposed two different laws of double refrac -ion, the one for Iceland spar. and the other for rock crystal. The last of these, founded on incorrect obser vations, has no existence in nature, while the first re presents the phenomena of Iceland spar, within the limits of the errors of observation.* The optical labours of Huygens were not limited to the subject of double refraction. His treatise on Diop
tries, which he began to compose in the early part of his life, but which was not published till after his death, is a work of transcendent merit, and was always espe cially admired by Newton himself. It contains a full account of the properties of lenses of all forms. The doctrine of spherical aberration. or that which arises from the spherical figure of the sut face, is treated with great clearness and ability. The subject of vision, and the remedy of its imperfections by lenses, is ably handled ; and the theory of telescopes, telescopic eye-pieces, and microscopes, is treated with fulness and perspicuity. The imperfect state of the telescope induced Huygens to extend his views beyond the rnere theory of its con struction. He employed himself in executin:_1,- the lenses with his own hands ; and by means of telescopes of 12 and 24 feet in focal length, which he himself made, he discovered the ring of Saturn, and the fourth of the satellites of that planet. Telescopes of the focal length of 86, 100, and 136 fcet, had been made by Campani of Bologna, by which Cassini discoveted the nearest satel lites of Saturn ; and Eustachio Divini, of Rome, had also constructed them of a great focal length. Huygens, however, made essential improvements in the method of using an object glass without a tube, which it was necessary to adopt in instruments of such extreme length. He placed the object glass in a short tube, at the top of a very long pole, so that this tube could turn in every direction by means of a ball and socket. The command of this tube at a distance was obtained by means of a silk string, so that he could bring it into a line with the axis of another short tube, containing- the eye-glass, which he held in his hand. By this means lie could direct his object glass in any azimuth, or to any altitude, and even to the zenith, if the pole were as long as the telescope. For this purpose he had a contrivance, by which Ile could raise or depress a stage that sup ported his object-glass. In this way Huygens used telescopes of enormous length ; and in one of his jour nies to England, he presented to the Royal Society two object glasses, one of which had a focal length of 120, and the other of 123 feet.
Having acquired great experience in the art of con structing telescopes, this eminent philosopher thought it right to transmit his methods to posterity, in a work published along with his Dioptrics, and entitled Com mentarii de formandis poliendisque vitris ad Telescopic. In this work he treats of the method of forming the gages, and the brass tools or moulds ; of the method of choosing proper glass ; of the grinding and smooth ing of the lens, and the method of giving it a perfect polish.
Along with this work appeared his Dissertaao de Coronis et Parheliis, a treatise of great ingenuity, in which he endeavours to give the theory of these cu rious and hitherto unexplained phenomena.