Mr. Howarp Gross on Great T elescopes of the Future. 9 
B 4.—All these matters of convenience mentioned before will be found to gain 
in importance as the size of the instrument is increased, 
Now I come to consider the practical difficulties in each case, and the most 
promising means of overcoming these difficulties. 
It may be said that the difficulty of manufacture is a question for the instrument- 
maker alone, and not to be discussed by those whose business it is to decide on the 
form of instrument employed, but it should be remembered that any advance in the 
size of Telescopes, Refractors or Reflectors, over those at present in existence, 
must be considered to be, to a certain extent, an experiment, and the nature of 
the difficulties which will be encountered, can at present only be speculated upon, 
even by the most experienced ; and therefore it behoves those whose province it is 
to decide on the matter, to inquire diligently into the relative practicability of the 
various forms of telescopes in order that they may not decide on a form which might 
be, if ever accomplished, of great usefulness, but which on trial would be found 
to be, in the present state of art, impossible to manufacture. 
With respect to Refractors, the first great difficulty to be met with is that of 
procuring suitable discs of glass. Of our glass manufacturers, only two firms seem 
to possess the secret of manipulation of optical glass, viz., Messrs. Chance, Brothers, 
and Company, of Birmingham, and M. Feil of Paris, a descendant of the celebrated 
Guinand. Of these, one at least speaks confidently of producing discs up to one 
metre in diameter ; but when I consider the difficulty which I know was experienced 
in moulding the 27-inch dises for the Vienna objective, I cannot say that I feel the 
same confidence. These 40-inch discs would require to be obtained in one single 
piece just three times the quantity of homogeneous glass that the Vienna discs 
required, and though Iam not of course in the secrets of the glass manufacturers, it 
appears to me that the chances of obtaining 40-inch discs in the present state of 
the art are remote. 
The other difficulties of manufacture of Refractors consist in the nicety of the 
operations connected with the calculations of the curves, the manipulation of such 
extremely costly material, and the enormous labour and trouble of the figuring and 
perfecting of the objective. All these, however, I have no doubt will be overcome 
by the optician for any size which the glass-maker is at all likely to produce. 
Now, as to the difficulties connected with the manufacture of Reflectors, whether 
metallic or silver on glass, 
First, as to the difficulty of producing the metallic or glass dise to work upon. 
Lord Rosse has succeeded years since in casting, annealing, and perfecting dises 
of six feet in diameter, and any difficulties he met with were not such as to lead us 
to the belief that the limit of possible size has been by any means reached. As 
regards glass mirrors, the question has never been discussed, for in any sizes that 
have been made up to the present time, it was only necessary to go to the plate 
glass manufacturers and say, “I want a disc of crown glass of such a diameter and 
C 
