I. Appendix to the description of the great Melbourne telescope

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 highn , 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 ofe , 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.