4 Mr. Howarp Gruss on Great Telescopes of the Future. 
any kind of suitable glass of six feet in diameter; but I may mention, that this 
is perhaps not to be much regretted, as the difficulty of preserving a silvered 
glass surface of large size would almost amount to an impossibility, and the 
process of resilvering, when the mirror is of large dimensions, becomes most 
formidable ; and, finally, it is probable that all the advantages of the silvered glass 
mirrors, without their corresponding disdvantages, may be secured by other means 
—a matter which I shall speak more of further on. 
Nore.—It may be remarked here that the bad conductivity of glass would render it very objectionable 
as a material for large mirrors. Professor Newcomb finds spherical aberration, arise from a slight 
difference of temperature of an objective only about four inches thick. How much greater would 
the effect be in case of a Reflector even of same thickness. (The effect would be fully four times as 
great as in a Refractor). Also, in consequence of the very perfect reflection of heat rays from the silver 
surface, the mirror is almost certain to dew every time the temperature of the air is lowered, and in 
large mirrors this dewing involves rapid destruction of the film of silver. 
4th.—General suitability for observatory work and measuring purposes. 
The same remarks apply to this as to the second point considered, viz., the 
greater permanence of the adjustments of the refractor ; indeed to a considerable 
extent one arises out of the other. It is necessary, therefore, to say but few 
words on the matter. 
It is not to be expected that very large telescopes are to supplant moderate sized 
instruments in what may be called the everyday work of an observatory ; such 
a course would be perfect waste of power; for if a telescope be completed of a size 
and power greater than hitherto attained, clearly every available moment should 
be made use of in using it for such objects and purposes as are beyond the reach 
of all other instruments ; and therefore this superiority of the Refractor is not to 
be considered of much weight in the case of very large instruments. 
5th.—In the Refractor there 1s no central mirror or arm to disturb the 
course of rays. 
There is no doubt that this has much to do with the difference of character of 
the image of a star as seen in Refractors and Reflectors. 
If, while looking at a star in a Refractor, we cause a diaphragm of about one- . 
sixth the diameter of the objective to be placed opposite its centre, we alter the 
character of the image to something very like that of the Reflector, excepting 
that we still have the secondary spectrum. It is difficult to describe the exact 
peculiarity, but those accustomed to the use of telescopes will understand what I 
mean. Which, then, is preferable? Here we have a wide difference of opinion, 
and very conflicting ideas, which, however, on analysis, may prove not so widely 
differing as at first sight appears. 
In the first place there is no doubt that good work has been done with both 
kinds of instruments ; secondly, in ninety-nine cases out of a hundred, it will be 
found that each observer will give his opinion as favourable to whatever kind 
of instrument he has himself been accustomed to work with. 
