1888.] President's Address. 53 



more about the history of its changes than could be got by observa- 

 tions extending over a lifetime if restricted to total eclipses. Some 

 observations were made during the partial phases of the last total 

 eclipse with the view of throwing light on the prospect of success. 

 Notwithstanding the unpromising nature of the results obtained, 

 I have reason for hoping that the desired object may yet be accom- 

 plished. 



In addressing you last year, that year which will be memorable as 

 the Jubilee of the reign of our beloved Sovereign, I alluded briefly to 

 the progress which science had made in the last half century, and 

 ventured to indicate one or two directions in which it seemed to me 

 possible that a very great addition to our physical knowledge might 

 some day be reached. I will not to-day venture to look so far ahead ; 

 but the mention of a total eclipse leads me to refer to some theories 

 now before the scientific world which are likely to undergo full 

 discussion and further examination in the near future, with the 

 probable result of a pretty general agreement as to their acceptance 

 >r rejection. 



It is now many years since Dr. Huggins discovered the peculiar 

 character of the spectra of the nebulae, spectra which he found to 

 consist mainly of bright lines, indicating that what we see is an 

 incandescent gas. The natural supposition to make at. the time was 

 that those distant masses of matter consisted of incandescent gas, of 

 which the luminosity was in some way kept up, probably as a result 

 of condensation. But the researches of Mr. Lockyer, as described by 

 him in the Bakerian lecture which he delivered last spring, and in 

 part in a previous paper communicated shortly before the last 

 anniversary, have led him to take a different view of the constitution 

 of nebulae. According to the theory advanced by him, the mass of 

 a nebula consists mainly of meteorites, which are constantly coming 

 into collision here and there ; and the glowing gas the existence of 

 which the spectroscope reveals, is merely a portion of the matter, 

 volatilised by the heat of collision. According to the former view 

 therefore, the nebula consists of glowing gas, not yet condensed into 

 a solid or liquid form, possibly in a condition even more elementary 

 than that of the so-called elements that we know on earth ; accord- 

 ing to the latter it consists mainly of discrete portions of solid matter, 

 and the glowing gas does not consist of the same matter perma- 

 nently glowing, but is continually supplied afresh by fresh collisions. 



A similar theory is applied to explain the self-luminosit}'- of the 

 nucleus, and sometimes the very root of the tail, of comets. A comet 

 is regarded as a swarm of meteorites, moving in orbits not greatly 

 differing from one another; and as the swarm approaches the sun 

 collisions become more frequent, and individually more potent, from 

 an increase in the velocities differential as well as absolute ; and 



