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XXXI. 
ON A MOUNTING FOR THE SPECULA OF REFLECTING 
TELESCOPES, DESIGNED TO REMOVE THE IMPEDI- 
MENT TO THEIR BEING USED FOR CELESTIAL PHO- 
TOGRAPHY AND SPECTROSCOPY. By G. JOHNSTONE 
STONEY, M.A., D.8c., F.R.S., Vice-President, Royal Dublin 
Society. 
[Received Aprit 17; Read Aprit 18; Published Junz 2, 1894.] 
I.—Intropvuction. 
Rer.ectine telescopes have conspicuous advantages over refractors, 
accompanied, however, by one defect which has greatly interfered 
with their usefulness. One of the advantages they have is that 
they are less costly. The glass of which specula are constructed 
need not be of good quality, and is therefore cheap : moreover the 
figuring of the one surface of the speculum of a reflector, although 
it requires to be effected with about six times the accuracy that is 
needed for any one of the four surfaces of the object-lens of a 
refractor, is easier and therefore less expensive than the very 
difficult task of combining the conditions which must be fulfilled by 
the four surfaces of a good object-glass. A speculum is therefore 
cheaper, perhaps about one-fourth of the cost of an equally good 
object-lens. But their great excellence lies in their optical per- 
formance; from their being absolutely free from the chromatic 
nuisance, and, moreover, from their spherical aberration, if corrected 
at all, being corrected for rays of all refrangibilities. This would 
give the reflector an overwhelming advantage over the refractor, 
and especially in photographing the heavens, or in using the spec- 
troscope, were it not for their one defect—that their line of colli- 
mation is apt to shift. Owing to this serious defect they can only 
be used for celestial photography and spectroscopy by persons 
whose powers of manipulation are altogether exceptional. 
The object-lens of a refracting telescope may be supported 
entirely by its edge, and is always so supported. This is suffi- 
cient, although, when the telescope is pointed towards the zenith 
