436 



On the great Melbourne Telescope. 



[June 11, 



out perfectly sound, but was a little '^in winding," and had Lord Rosse's 

 " Crowsfeet " on part of its surface. These might have been ground out, 

 but Mr. Grubb preferred recasting it. It was broken by an iron ram of 

 70 lbs. falling four feet. When the blows were applied through a bar of 

 wood, they did nothing ; but the first one through iron broke it into four 

 equal pieces. These, when put together, fitted exactly, showing that 

 there was no unequal shrinkage in cooling. Some changes were made 

 before the second casting, on Sept. 22. The hoop-bed of the mould was 

 much strengthened ; the core, which formed the central aperture, and 

 seemed to interfere with the regular flow of the metal, was raised 1|- inch 

 above the mould ; and this last set on a strong cast-iron frame, which 

 could be inclined at pleasure. It was sloped, so that at the first part of 

 the pouring the melted alloy filled it from its lower edge to its middle ; it 

 was then lowered rapidly to be horizontal while the pouring was com- 

 pleted. When it came from the oven, the central disk, which was under 

 the core, was cut out. This, as well as the third casting, which was made 

 November 24, came out perfect. 



A description is given of the polishing machine, which is remarkable 

 for its smooth and equable action, and not less so for the facility it gives 

 of testing the figure of the speculum during the polishing. Removing the 

 pohsher (which is built up of wood, so as to combine great strength and 

 lightness), one man, by turning a winch, sets the speculum upright. Doors 

 are opened, and by an eyepiece properly placed a dial or artificial star at a 

 suitable distance is examined. The specula were ground flat at their back, 

 and true at their edges ; and their focal lengths difl"er only 1| inch. 



Not less important than the perfect figure of a speculum is its being so 

 supported both at the back and edge that it may be subjected to no irre- 

 gular pressure, for to such it is almost inconceivably sensitive. The ar- 

 rangements to secure this are very elegant, and have proved entirely suc- 

 cessful. 



The lattice of the tube consists of steel bars 5 thick, and, on average, 

 2j broad, forming openings of 1 7 by 9 ; it is very light, and so stifle that 

 1 cwt. produced only a deflection -g-J-jj inch at 20 feet. The mode of sup- 

 porting the small specula is described, and the mechanism by which the 

 moving of them for focal adjustment is made easy to the observer. One 

 of these specula is peculiar ; it is an achromatized lens, whose coincident 

 surfaces are cemented ; its front one so curved that there can be no false 

 image formed by it, its fourth coated pretty thickly with silver. This will 

 be more permanent than the metal, and is expected to give more light. 



The equatorial is worthy of the telescope. By inverting the usual 

 arrangement of the polar axis, the eyepiece, circles, and centre of gravity 

 of the whole are brought near the ground ; each axis is provided with three 

 sets of counterpoises ; the polar with two, which relieve its pivots of 

 lateral pressure ; the third lightens the end pressure of the lower one. 

 Those of the declination axes act, two in its plane parallel and perpendicular 



