1868.] 



great Melbourne Telescope. 



435 



elusion that the equivalent Achromatic will certainly not be less than 

 3 feet, and may be much more. He adduces experimental facts in support 

 of this, and points out the conditions which are necessary for a fair com - 

 parison of the two kinds. 



3. The tube was to be metal lattice-work. This plan, originally pro- 

 posed by Sir J. Herschel, had been tried by Lord Rosse, Mr. Lassell, and 

 Mr. W. De la Rue, with marked advantage ; its main purpose is to dimi- 

 nish the evils caused by currents of unequally heated air eddying in the 

 tube. 



4. They decided that the speculum should be metal, and not silvered 

 glass. It seemed imprudent to risk the success of the undertaking by 

 venturing on an experiment whose success was not assured ; it was not 

 known whether the silver could be uniformly deposited on so large a 

 scale ; some facts appear to show that glass is more liable to irregular 

 action than speculum metal ; and the intensity of the light in these tele- 

 scopes is not as great as had been expected. 



5. The telescope was to be a Cassegrain, not a Newtonian ; and this 

 was the result of long discussion. This form is little known, for Newton's 

 hostility to it has created a prejudice against it. Its chief defect is the 

 difficulty of getting a magnifying power so low that the eye can only take 

 in the whole pencil, as the first image is magnified from five to six times 

 by the second speculum. This requires a huge eyepiece, of which the 

 lenses are costly, and, by their thickness, intercept light. One defect 

 attributed to it by Newton proves to be an advantage ; he thought that 

 the reflection of metal is like that of glass, faintest at perpendicular inci- 

 dence, but increasing in intensity with the obliquity ; therefore brighter in 

 his telescope at 45° than in the other nearly perpendicular. But it is now 

 known that the law is different, so that the reflection at 45° is ^^^^ 

 powerful than the other. The chief advantage of the Cassegrain, and 

 that which influenced the Committee, is its extreme convenience to tlie ob- 

 server ; he is near the ground, and has to move through a small space to 

 command the whole sky, instead of standing on a structure nearly 40 feet 

 high, which cannot be used without fatigue and even danger. 



Then formulae are given for finding the constants of the telescope ; the 

 foci of its specula are 366 and 75 inches, its lowest power 240, and its 

 extreme field of view 14'-3. 



The composition of the specula is Lord Rosse' s, four equivalents of 

 copper and one of tin, respecting which much detail is given ; and still 

 more of the process of casting the first speculum, which took place July 

 3, 1866. This was managed according to the method of Lord Rosse, 

 with some modification, caused by the necessity of having a projecting 

 band round the edge of the speculum, and an aperture in its centre. It was 

 conducted without accident, and safely transferred to the annealing-oven, in 

 which a thermocouple of platinum and iron was inserted, which, at the 

 end of twenty-four days, showed that it was completely cooled. It came 



VOL. XVI. 2 Q 



