I 



TRANSACTIONS OF SECTION A. 661 



severely criticised by the author, tbe general principle of their mounting being 

 ' telescopes are always supported at the middle, not at the ends.' 

 ' Every part is, if possible, supported by counterpoises.' 



' To these principles everything is sacrificed. For instance, in an equatorial 

 the polar axis is to be supported in the middle by a counterpoise. This not only 

 makes the instrument weak (as the axis must be single), but also introduces some 

 inconvenience into tbe use of it. The telescope is on one side of the axis ; on the 

 other side is a counterpoise. Each end of the telescope has a counterpoise. A 

 telescope thus mounted must, I should think, be very liable to tremor. If a 

 person who is no mechanic and who has not used one of these instruments may 

 presume to give an opinion, I should say that the Germans have made no im- 

 provement in instruments except in the excellence of the workmanship.' 



1 have no doubt that this question had often occupied Airy's mind, for in the 

 Northumberland Equatorial Telescope which he designed shortly after for Cam- 

 bridge he adopted what has been called the English form of mounting, where the 

 telescope is supported by a pivot at each side, and a long polar axis is supported at 

 each eixi. This telescope is in working order at the present time at Cambridge. 



When he became Astronomer Royal he used the same design for what was for 

 many years the great equatorial at Greenwich, though tlie wooden uprights form- 

 ing the polar axis were in the Greenwich telescope replaced by iron. It says much 

 for the excellence of the design and workmanship of this mounting, designed as it 

 was for an object glass of about 13 inches diameter, when we find the present 

 Astronomer Royal, Mr. Christie, has used it to carry a telescope of 28 inches 

 aperture, and that it does this perfectly. 



Notwithstanding the greater steadiness of the English form of mounting, 

 the German iorm has been adopted generally lor the mountiDg of the large 

 refractors recently made. 



There is much interesting matter in this report of an historical character. 



As I have already said, the new astronomy, as we know it, did not exist, but 

 in a report ' on optics, in the same volume, by Sir David Brewster, we find that 

 spectrum analysis was then occupying attention, and the last paragraph of this 

 report is well worth quoting : ' But whatever hypothesis be destined to embrace 

 and explain this class of phenomena, the fact which I have mentioned opens an 

 extensive field of inquiry. By the aid of the gaseous absorbent we may study 

 with the minutest accuracy the action of the elements of material bodies in all 

 their variety of combinations, upon definite and easily recognised rays of light, 

 and we may discover curious analogies between their affinities and those which 

 produce the fixed lines in the spectra of the stars. The apparatus, however, which 

 is requisite to carry on such inquiries with success cannot be procured by indivi- 

 duals, and cannot even be used in ordinary apartments. Leases of large diameter, 

 accurate heliostats, and telescopes of large aperture are absolutely neces.sary for 

 this purpose ; but with such auxiliaries it would be easy to construct optical com- 

 binations, by which the defective rays in the spectra of all the fixed stars down to 

 the tenth magnitude might be observed, and by which we might study the effects 

 of the very combustion which lights up the suns of other systems.' 



Brewster's words are almost prophetic, and it Avould almost appear as if he 

 unk-nowingly held the key to the elucidation of the spectrum lines, for it was not 

 until 1859 that KirchhoH''s discovery of the true origin of the dark lines was made. 



Frauiihofer was the first to observe the spectra of the planets and the stars, and 

 to notice the different types of stellar spectra. In 1817 he recorded the spectrum 

 of Venus and Sirius, Hud later, in 1828, he described the spectrum of Mars; also 

 Castor, Pollux, Capella, Betelgeux, and Procyon. 



Fraunhofer, Lament, Donati, Brewster, Stokes, Gladstone, and others 

 carried on their researches at a time when the principles of spectrum analysis 

 were unknown, but immediately upon Kirchhofl"s discovery great interest was 

 awakened. 



With spectrum analysis thus established, aided as it was later by the greater 

 development of photography, tht new astronomy was firmly established, 



' Brit. A?soc. llej?,, 1831-32, p. 308 



