ADDRESS. 31 



Thus the floor of heaven is not only ' thick inlaid with patines of 

 bright gold,' but studded also with extinct stars; once probably as brilliant 

 as our own sun, but now dead and cold, as Helmholtz tells us that our sun 

 itself will be, some seventeen millions of years hence. 



The connection of Astronomy with the history of our planet has been 

 a subject of speculation and research during a great part of the half- 

 century of our existence. Sir Charles Lyell devoted some of the opening 

 chapters of his great work to the subject. Haughton has brought his 

 very original powers to bear on the subject of secular changes in climate, 

 and CroU's contributions to the same subject are of great interest. Last, 

 but not least, I must not omit to make mention of the series of massive 

 memoirs (I am happy to say not yet nearly terminated) by George Dar- 

 win on tidal friction, and the influence of tidal action on the evolution of 

 the solar system. 



I may perhaps just mention, as regards telescopes, that the largest 

 reflector in 1830 was Sir W. Herschel's of 4 ft., the largest at present 

 being Lord Rosse's of 6 ft. ; as regards refractors the largest then had a 

 diameter of 11|^ in., while your fellow-townsman Cooke carried the size 

 to 25 in., and Mr. Grubb, of Dublin, has just successfully completed 

 one of 27 in. for the Observatory of Vienna. It is remarkable that the 

 two largest telescopes in the world should both be Irish. 



The general result of astronomical researches has been thus eloquently 

 summed up by Proctor : — ' The sidereal system is altogether more 

 complicated and more varied in structure than has hitherto been 

 supposed ; in the same region of the stellar depths co-exist stars of many 

 orders of real magnitude ; all orders of nebulte, gaseous or stellar, 

 planetary, ring-formed, elliptical, and spiral, exist within the limits of the 

 galaxy ; and lastly, the whole system is alive with movements, the laws 

 of which may one day be recognised, though at present they appear too 

 complex to be understood.' 



We can, I think, scarcely claim the establishment of the undu- 

 latory theory of light as falling within the last fifty years ; for 

 though Brewster, in his ' Report on Optics,' published in our first volume, 

 treats the question as open, and expresses himself still unconvinced, 

 he was, I believe, almost alone in his preference for the emission theory. 

 The phenomena of interference, in fact, left hardly any— if any — 

 room for doubt, and the subject was finally set at rest by Foucault's 

 celebrated experiments in 1850. According to the undulatory theory 

 the velocity of light ought to be greater in air than in water, while if 

 the emission theory were correct the reverse would be the case. The 

 velocity of light — 186,000 miles in a second — is, however, so great that, 

 to determine its rate in air, as compared with that in water, might 

 seem almost hopeless. The velocity in air was, nevertheless, determined 

 by Fizeau in 1849, by means of a rapidly revolving wheel. In the 

 following year Foucault, by means of a revolving mirror, demonstrated 



