THE GREAT MELBOURNE TELESCOPE. 
135 
which r is found =02051, and thence finally f . Omitting the effect of r, 
the field of view is in general 5= COt *^ 1 ; hence the constants of the telescope are 
Large speculum E=366 inches 
A= 48 „ 
Small speculum /= 74-71 
a= 8-05 
d= 61 
d'= 332-31 
- d = 5-4477 
Lowest power f'= 16-10 
Distance of eyestop from 2nd lens 2*89 
M=240 
Q= 14'-32 
Equivalent focus 1994 
As soon as these were decided on, the contract (which, beside the telescope and a 
duplicate large speculum, included an apparatus for polishing and a steam-engine for 
working it) was signed at the close of February 1866, and the preparations for casting 
the specula were urged rapidly forward. All had been so carefully considered by Mr. 
Grubb, that scarcely in any instance was there any necessity to change what he had 
planned. While these were going on, much progress was made with the more massive 
parts of the equatorial, and the polishing-machine was got ready. The alloy for the 
specula was also prepared. It is that of Lord Rosse, four equivalents of copper to one 
of tin ; it possesses more power of resisting tarnish than those which deviate a little on 
either side of this proportion, and is probably as reflective as any. The liability of good 
speculum metal to tarnish has been much exaggerated. I have elsewhere given some 
proofs of this, and may add that there is at the Armagh Observatory a six-inch Gregorian 
by Short, bearing the date of 1745, which is nearly as bright as at first. As to this 
Mr. Lassell and Mr. W. De La Rue are good witnesses, especially the latter ; for his 
13-inch showed no signs of tarnish, though it was for several years exposed to the influ- 
ence of photographic chemicals, some of which are very bad neighbours for polished 
metals. Like most other brilliant metals, riot excepting silver, it reflects the less 
refrangible rays in greater proportion than the others, which gives stars a tendency 
towards red or orange in reflecting-telescopes. This, perhaps, may not be the case with 
an alloy proposed by Rev. William T. Kingsley, who adds to the above compound 
one-fourth of an equivalent of zinc. A small piece of this, with which that gentleman 
MDCCCLXIX. t 
