XXV. Observations on the Nebulæ

In laying before the Royal Society an account of the progress which has been made up to the present date in the re-examination of Sir John Herschel’s Catalogue of Nebulæ published in the Philosophical Transactions for 1833, it will be necessary to say something of the qualities of the instrument employed. The telescope has a clear aperture of 6 feet, and a focal length of 53 feet. It has hitherto been used as a Newtonian, but in constructing the galleries provision was made for the easy application of a little additional apparatus to change the height of the observer, so that the focal length of the speculum remaining the same, the instru­ment could be conveniently worked as a Herschelian.