Mr. Howarp Gruss on Great Telescopes of the Future. 3 
3rd. The next advantage of Refractors we have to consider is that of the 
Greater permanence of the optical portion. 
This is an advantage the weight of which much depends upon local circumstances. 
If the telescope was to remain the property of, and to be worked by the person 
who completed the optical part (as in the case of Lord Rosse, and Mr. Lassell, and 
Mr. Nasmyth), this disadvantage of Reflectors would not have much weight ; and 
it is for this reason, probably, that it has been remarked (and with some truth too) 
that Reflectors very seldom do good work except in the hands of their makers. If, 
however, the telescope is to be sent to a foreign country this point becomes worthy 
of serious attention, and its advantage becomes more and more decided as the 
situation of the telescope becomes more and more inaccessible. There are then 
only the following safe methods of managing a Reflector :—In any case there should 
not be less than two mirrors. If it be a metallic mirror, either a person should be 
sent out with the telescope duly instructed in the art of figuring the mirror, or a 
relay of several mirrors, three or four (according to position of observatory and time 
taken to reach it), should be provided, and each mirror as it becomes tarnished, sent 
back to the maker to repolish ; or, in the case of a silvered glass mirror, apparatus 
should be provided and a suitable operator permanently appointed to resilver the 
mirrors whenever required ; which in large telescopes would be very often indeed. 
It is not, however, to be taken for granted that all objectives are perfectly free 
from deteriorating influences. The 13-inch objective in Greenwich, I believe, 
requires periodic cleansing of its surface to free it from a peculiar tarnish which 
attacks the flint (probably the lead used in the flint has much to do with this) ; 
and we hear of object glasses which have lost a large per-centage of their trans- 
parency from this peculiar tarnish. As regards metallic Reflectors, there seems 
much difference of opinion as to their durability, arising probably from the fact 
that the liability to tarnish increases very rapidly as the quality of the alloy 
becomes “low,” z.e., as the proportional quantity of copper is increased above the 
true atomic proportion of four to one. A slight addition to the quantity of copper, 
though it may be hardly appreciable as to colour while the polish is fresh, increases 
enormously the liability to tarnish, I have seen a mirror whose polish was 
perfectly good after thirty years, though no very particular care was taken of it. 
As mirrors increase in size, however, of course the difficulties of preserving 
them become much greater; but it would be fair to assume that a large speculum 
of good alloy should, in a good climate, stand two or three years’ constant work 
before requiring repolishing, and by our present system of polishing, it is possible 
to repolish mirrors, without any necessity for refiguring, with almost absolute 
certainty. 
As regards silver on glass mirrors, it is hardly worth discussing their relative 
powers of permanence, as at the present date the art of glass-making has not arrived 
at that degree of perfection that will permit the makers to undertake discs of 
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