10 Mr. Howarp Gruss on Great Telescopes of the Future. 
such a thickness,” and forthwith the glass dise was delivered without any trouble ; 
but, when we come to these extraordinary sizes, it is quite a different matter. For 
the 4-foot disc of glass for the Paris Reflector, in place of that which has so recently 
resulted in failure, the St. Gobain Glass Company require twelve months’ time to 
perfect (although, be it remembered, the quality of the glass is here of no consequence 
whatever); and I have been myself in correspondence with the principal glass manu- 
facturers here and on the Continent, and not one of them are willing to undertake 
even a 6-foot glass disc; so that it would appear that, above that size, the silver on 
glass mirrors are out of the question. 
This much, however, is to be said—If anyone were to go to a brass or bell-founder’s 
and ask them to undertake a speculum of six feet in diameter, they would almost 
certainly be met with a refusal ; and yet Lord Rosse has proved the feasibility of it. 
And so, reasoning by analogy, might the manufacture of a six or eight foot glass 
mirror be possible, if undertaken in the same scientific spirit that Lord Rosse under- 
took his. I answer to this—Yes ; perfectly true ; but this is too purely a specu- 
lative matter to be considered at the present day in the choice of telescopes. 
The other great difficulty in the manufacture of Reflectors is the annealing of the 
disc, and I believe it is this difficulty which limits to so narrow an extent the 
production of glass discs for silver on glass mirrors. 
I should wish to say a few words on this matter of annealing. 
Anyone who has studied the matter knows that if it were possible to ascer- 
tain how the disc of metal, which is built up im the oven, is cooling—what part is 
cooling quickly, what part is cooling slowest, and to have the power of control- 
ling the rate of cooling of the different parts—that the problem of annealing a 
disc of almost any size would be solved. By a disc I mean a circular plate whose 
diameter bears a large proportion to its thickness. 
But when the disc is built up with solid brickwork several feet thick, we cannot 
tell what is going on. It may be cooling quickly round the edge, and the centre 
in cooling drags, so to speak, from this solid ring on the outside, and either comes 
out of the oven cracked, or ready to crack at the first slight disturbance of its 
molecules; or a draught of air may be creer in between chinks in the bricks, 
and doing mischief. 
In the case of the Melbourne Reflector the Rev. Dr. Robinson proposed the 
introduction of a small thermo-pile into the oven, which enabled us to get the rate 
of cooling, and enabled me on one occasion to detect a draught of cold air from a 
crack in the bricks, which if undetected might have caused the loss of the mirror. 
I shall now in a few words describe the method I would propose for the annealing 
of large discs either of glass or speculum metal. 
The figures A B C and D represents vertical and horizontal sections of an oven 
very similar to that used for annealing the Gt. Melbourne mirrors, slightly modi- 
fied, howeyer, to suit the larger sizes. 
