1870.] 



Great Melbourne Telescope. 



219 



aperture observations at low altitudes are almost impracticable, and would 

 probably never have to be resorted to except in the case of comets. The 

 roof itself cuts off some of the range near the subpolar meridian ; this, 

 again, is not likely to be of much consequence. 



The steam-engine, polishing-machine, and crane have been mounted in 

 the room devoted to them ; this room adjoins and is on the same floor 

 (raised 4 feet from the ground, and 3 to 6 feet from the floor of the other 

 rooms) as the telescope-room. To the east end of this machine-room, 

 and communicating therewith, a small t lean-to boiler-house has been 

 added ; in the west wall is a window which, when open, will leave sufficient 

 clear space to admit of viewing a distant nearly horizontal object for the 

 purpose of testing the mirrors. 



The large speculum (A) was originally attached to the tube in its var- 

 nished condition ; on the first favourable occasion it was taken down and 

 unvarnished — a process which proved more troublesome than had been 

 anticipated. The lac was very refractory, and the difficulty of removal ex- 

 aggerated by the extreme heat then prevalent ; after a process of solution 

 in alcohol, mopping up, and washing with water frequently repeated, although 

 there seemed no lac which would still dissolve, a large number of markings 

 caused originally by the varnish brush were apparent, and the whole 

 surface had an unpleasant mealy appearance. 



It was thought, however, that the light lost would not prove serious, 

 and in any case it did not seem that any further operation except polishing 

 would improve matters ; the speculum was therefore remounted and tried ; 

 and although it was of course impossible to say what would have been the 

 effect of a more perfect polish, the views given of the brighter nebulae were 

 grand in the extreme, and left nothing to be desired. 



By degrees, however, and without much exposure, the surface became 

 more and more tarnished, with evident effect on the performance. 



In the meantime the second mirror (B) had been unvarnished ; in this case 

 naphtha was used as the solvent, the solution mopped up, and the surface 

 washed with soap and water. After a frequent repetition of this process, the 

 surface seemed clear of impurities, and though not so bright as I had fre- 

 quently seen it in Mr. Grubb's workshop, there were no signs of mealiness, 

 the only unpleasant casualty being a considerable pitting of two patches 

 some two inches square, produced by droppings from the muriate used in 

 soldering the tin cover. These pittings are deep and unsightly ; but the 

 extent of surface corroded is comparatively so small that the effect must 

 be inconsiderable. 



The specula were exchanged about two months ago, and A put on the 

 machine ; but nothing has yet been done towards repolishing, as the ne- 

 cessary arrangements have not been got together for performing that delicate 

 operation with due convenience. 



Of work done, I cannot yet speak with any satisfaction since it became 

 at all practicable to use the telescope ; the history which I have to relate is 



