New Achromatic Lenses.

The Manufacturer and Builder 6, 1875

Vernon Harcourt had, before his death, glass disks prepared, intended for telescopic objectives, in which all possible imperfections were to be corrected. They were to be made of lenses of phosphatic glass, containing a proper amount of titanic acid, while the achromatism was obtained by a flint lens containing a large amount of terborate of lead. As a single lens of this kind would require too strong a curve, he contemplated having two convex crown lenses, with a concave flint of terborate of lead, a system which had been also accepted by Dollond, as may be seen in the old telescopes and spyglasses of that celebrated maker. Harcourt, recogniting that in case the back lens had at the most one-third of the total power of converging the rays, the surfaces could be placed in contact without preventing a perfect correction for spherical and chromatic aberrations. This allows the cementation of the lenses together with Canada balsam, and so protects the borate of lead glass, which is very soft, and has the defect of being easily tarnished.

Harcourt had left two glass disks of titanium glass, and two others of terborate of lead glass. These disks were confided to Mr. Grubb, who found that unfortunately one of them had threads and could not be used. Mr. Stokes then replaced that lens by a lens of crown glass, of which the convergent power was about one-eighth of the total convergent power, so that the secondary dispersion was not entirely annuled, and thus he obtained a good telescope. It had 24-inch diameter, 28 inches focal light, and notwithstanding it was not perfect, it demonstrated in a sufficient manner the correctness of the principle that the original inventor, Harcourt, wished to apply.