CHAP. III., 6.] ASTRONOMY. LORD ROSSE HENDERSON AND BESSEL. 



65 



(299.) 



jord Rosse 



disappeared. The last two hundred years have not 

 presented any such astonishing phenomena as the 

 new stars recorded in the sixteenth and seventeenth 

 centuries ; but singular variations in the brightness 

 of some of the most conspicuous stars, as a Orionis 

 and ?; Argus, have been discovered by Sir John 

 Herschel. Several stars of short and irregular 

 periods of varying brightness were recorded towards 

 the close of last century. But upon this very in- 

 teresting subject I must content myself with refer- 

 ring to the details given by Professor Argelander in 

 the third volume of Humboldt's Cosmos. 



EARL OF ROSSE. Latest observations on Nebulae. 

 j j g a remarkable circumstance, that as the reflect- 

 i n telescope was a British invention, so the more 

 elescopes. important improvements and applications of it have 

 been almost confined to the United Kingdom. It is 

 also worthy of notice that the manufacture has pros- 

 pered more in the hands of amateurs than of regular 

 opticians. Sir William Herschel appeared at one 

 time to have brought the invention to its highest per- 

 fection, but the Earl of Rosse has made an important 

 step farther ; not only by constructing a larger tele- 

 scope than had been made before, but by adapting 

 machinery driven by steam power, to the grinding 

 and polishing of the mirror ; so that the largest 

 speculum may be finished with nearly the same ac- 

 curacy and expedition as the smallest. The chef 

 d'&uvre of Lord Rosse is a telescope of 6 feet aper- 

 ture, and 53 or 54 feet of focal length. It was com- 

 pleted in the latter part of 1844, and erected at Par- 

 sonstown in Ireland. 



(300.) Let me here record the important fact, that neither 

 Bculties ran k nor wea lth could absolve Lord Rosse from those 

 toils and disappointments which attend all new and 

 original efforts. Thereis no royal road to such triumphs. 

 The Irish nobleman owes his success entirely to his 

 unwearying perseverance and mechanical skill. Even 

 his assistants were countrymen instructed by him- 

 self in his own workshops, where the very steam-en- 

 gine which drives the polisher was fabricated. His 

 labours to improve the telescope date from 1828 

 (when he was Lord Oxmantown), or even earlier, and 

 they appear to have been unremitting until 1 844 ; in- 

 deed I might say until the present time. Com- 

 mencing with a variety of ingenious attempts to cor- 

 rect spherical aberration, and to overcome the ex- 

 treme difficulty of procuring large castings of so ex- 

 cessively brittle a material as speculum-metal, they 

 terminated in the rejection of all minor helps and 

 expedients, and in the fortunate completion of im- 

 mense mirrors at a single casting, and of correctly 

 parabolic figure when ground and polished. The 

 speculum of his largest telescope weighs four tons. 

 It was polished in six hours, and its surface is con- 



ncoun 

 2red. 



siderably more than twice as large as that of Sir 

 William Herschel's forty-feet instrument. 



We cannot enter into the details of the methods, (301.) 

 which evince no small mechanical skill and scientific Methods of 

 ingenuity, together with a perseverance admirable in ^ 

 itself. 1 I will only mention how the upper and 

 lower surfaces of the casting were made to cool nearly 

 equally fast. To effect this the lower surface of the 

 mould (which naturally retains the heat more than 

 the upper) was made of iron, a good conductor, whilst 

 the upper surface was made of sand. This effected 

 the purpose; but it being found that air bubbles 

 entangled in the fluid metal could not escape beneath, 

 and injured the casting, the iron bed was constructed 

 of hoops set on edge and closely packed, the crevices 

 allowing the escape of air, whilst the cooling pro- 

 ceeded as before ; and this ingenious contrivance was 

 perfectly successful. 



Many difficulties in detail have been found in the (302.) 



mounting and use of so gigantic a mass, particularly Applica- 



<?.i j- . <?.r i a tion to Ne- 



on account of the distortion or the mirror by flexure. bulae and 



But these have gradually been surmounted by Lord its results. 

 Rosse. His published observations (Philosophical 

 Transactions, 1850) relate almost entirely to objects 

 of the class of nebulae ; and as I cannot enter into 

 details, I may state the general results in two or three 

 sentences. (1.) As might have been expected, many 

 nebulae which resisted the power of former telescopes 

 (for, except in rare instances, nothing greater than 

 eighteen-inch apertures have been directed to them) 

 have been "resolved" into stars by the six-feet specu- 

 lum. (2.) The aspect of a great number of nebulae 

 described by the two Herschels is materially modified 

 by the power of the telescope to embrace the fainter 

 prolongations of these singular objects. In general, 

 the symmetrical forms are very much cut up and 

 confused, and in many cases vanish altogether. 

 (3.) Instead of these, a certain species of symmetry, 

 of a vague yet very remarkable description, has been 

 detected by Lord Rosse, probably for the first time. 

 It is a spiral arrangement of the nebulous coils 

 round a centre, resembling somewhat the spiral 

 emanations of revolving fireworks. The well-known 

 nebula No. 51 of Messier's Catalogue shows this 

 in a remarkable manner. 



Some observations have been made upon the moon. (303.) 

 It is much to be desired that these were continued, 

 and that the planets could also be observed ; but I 

 believe that the climate of Parsonstown affords but 

 few nights favourable to observation. 



HENDERSON AND BESSEL. Parallax and distance (304.) 

 of the Fixed Stars. THOMAS HENDERSON, at one He . n . d ?5 s n 



r\ * r\ j nis " lr "J 



time government astronomer at the Cape ot U-ood and charac , 

 Hope, and subsequently professor of practical as-ter. 

 tronomy in the university of Edinburgh, and his 



1 For a comparison of Lord Rosse's and Messrs Naysmith and LasselPs (subsequent) methods of mechanical grinding and 

 polishing, see Astron. Soc. Notices, vol. ix. p. 110. In Lord Rosse's apparatus every stroke of the polisher is almost a straight 

 line ; in Mr Lassell's it never is. 



