I. Appendix to the description of the great Melbourne telescope

Abstract

Since this paper was read the author has made several observations of the quantity of light transmitted by object-glasses, and determined the index of absorption in various specimens of glass. The results of some of these are in accordance with the opinion expressed in the paper; but others present a difference which is very satisfactory as indicating a sur­prising progress in the manufacture of optical glass. The observations were made by means of Zollner’s photometer. The following results were obtained for the intensity of the light trans­mitted by a variety of object-glasses :— Of the above, a belongs to the Armagh Observatory; it is by one of the Dollonds, older than 1790, and is probably one of their first attempts at a triple combination, b is the original object-glass of the Armagh circle; it was made by Tulley about 1828 ; the crown is greenish, and is supposed to be English; the flint is believed to have been from Daguet. c was made for the author by Tulley in 1838; its glass is French, the crown is greenish, d is by Cauchoix; the crown is greenish, and has probably a high n , but its mean thickness is only 0.39. e is by Messrs. Cooke; the glass is Chance’s, f is by Grubb, the glass Chance’s: the very high trans­mission of this lens is in part due to the cementing of the adjacent sur­faces, which, while it makes more difficult the correction of spherical aberration, removes almost entirely the reflection at a surface of crown and one of flint: the factor for this =0.9036; and if the I be multiplied by this, we obtain 0.7806, nearly that of e , the difference being due to the reflection at the film of cement, g is also by Grubb, and cemented; the glass is by Chance, h is by Fraunhofer.