31r. Ross has called attention to a defect in optical glom, which may be detected by the searching agency of polarised light. A glass of not more than 6 inches in diameter, undergoes the annealing process with ' difficulty, cooling moro rapidly at the surface than in the interior, and as this tendency increases with the size, the production of a disc of 29 inches is justly regarded as a very remarkable work.
Sir John Herschel, in a recent article on the telescope (' Ency.
suggests that the ultimate perfection of the achromatic telescope would be obtained were it possible to manufacture other species of glass with a much lower dispersive power than crown or plate glass, or a different action on the rays of intermediate refrangibility. The fluoric compounds have remarkably low dispersive power, tlmt of cryolite and fluate of lime is only two-thirdd.that of plate glass, holding nearly the same place with respect to this glass that the latter does to flint. The fluoride of strontium is also suggested as a promising material. It is also stated that M. Jamin, by using oxide of zinc instead of that of lead in flint glass, has succeeded in forming a zinc flint glass of exquisite limpidity, of low refractive and dispersive power, and capable of being wrought into discs of any size.
Attempts have been made by M. Chevalier to diminish the aberra tions by means of two achromatic object-glasses placed at a certain distance from each other in the tube ; and by Mr. Rogers of Leith, by a single convex lens of plate-glass, in combination with a double achro matic lens, the convex lens being of plate-glass, and the concave lens of flint•glass. This last gentleman proposes to unite the red and violet rays at the image of the object by a proper distance between the single and the double lens, and to correct the spherical aberration either by giving proper curvatures to the surfaces of the compound lens, or by placing the two lenses at a email distance from each other in the narrower part of the converging cone of rays. (' Memoirs of the Astron. Soc.; vol. iii) This was reduced to practice in 1830 by M. PRisal, of Vienna, under the name of the Dialytic Telescope, which is characterised by Sir John Herschel as "a very artificial and beautiful invention, highly deserving further triaL" Somewhat resembling this contrivance is Mr. Peter Barlow's plan of placing in the narrowing cone of rays from
a plate-glass object-lens a concave lens formed of two plate-glass cap sules of equal thickness, inclosing between them a highly dispersive fluid, namely, the bisulphide of carbon, of which the refractive index is and the dispersive power 0.115, or more than double that of flint-glass. This fluid lens is concave-convex. By proper curves tho spherical aberration can be destroyed; and by varying the distance between the two lenses, exact achromaticity can be produced. The principle was tested in a telescope of 8 inches aperture and 12 feet focal length, with success: as Mr. Barlow remarks," less than an ounce of sulphuret of carbon, value three shillings," was made to perform the office of a very costly flint disc of 8 inches. (' Phil. Trans., 1828, 1829, 1831.) Sir D. Brewster has suggested (` Treatise on New Phil. Inst.; p. 400) that it may be possible to remove, or at least very much diminish, the uncorrected colour in the image by the use of two lenses of the same kind of glass with the same or different dispersive powers. He pro poses that the exterior lens should have the meniscus form, the convex side being outwards, in order, from the obliquity of the incident rays to the surface, that the dispersion produced by that lens may increase in a higher ratio than its refraction, so that the dispersion produced by the other lens may be corrected ; while in each pencil the rays, after refraction through both, may be convergent.
It would be improper to omit here to mention that M. Amici, at Modena, some years since, invented a species of achromatic telescope by a combination of four prisms, all of the same kind of glass : the refracting edges of one pair of the prisms were parallel to one another, and those of the other pair were also parallel to one another, but perpendicular to the edges of the first pair; and each pair formed an achromatic combination. By the refraction in the first pair, the breadth of the object is magnified, and by that in the second pair the length is magnified in the same ratio : thus the result is an image undistorted and magnified. Sir John Herschel states that, in 1826, he saw in the hands of its inventor one of these telescopes, which magnified about four times.