THE PEESIDENX'S ADDEESS. 323 



positive from this plate is beyond description. In the small 

 portion of the sky containing the splendid nebula in Andromeda, 

 Mr. Barnard has estimated that in the original negative (8 by 

 10 inches) 64,000 stars can be distinctly counted. These mag- 

 nificent delineations of the invisible heavens are entirely due, as 

 I have stated, to the adoption of the dry-plate process in photog- 

 raphy, which admits of an exposure to the sky for a considerable 

 length of time, without the plate becoming in the least deterior- 

 ated. In the days of the collodion wet-process, such closely 

 studded photographs could not have been possible. 



Some very beautiful photographs of nebulse, taken by Mr. 

 A. Ainslie Common, F.R.S., of Ealing, especially of that of the 

 celebrated nebula in Orion, are remarkable for the excellent 

 definition of the most delicate portions, and they have excited 

 the admiration of those who have had an opportunity of inspect- 

 ing them. For these exquisite photographs, Mr. Common was 

 awarded the gold medal of the Royal Astronomical Society. Dr. 

 Huggins, in his spectroscopic investigation of the Orion nebula, 

 has also employed photography with most interesting and sug- 

 gestive results. The more closely that he has examined this 

 nebula, and others of the same class, the stronger appears to be 

 his conviction that these filmy objects are celestial systems in 

 various stages of evolution, standing possibly in, or the 

 beginning of, the evolutionary cycle, and consisting principally 

 of gas of a high temperature and very tenuous. On the other 

 hand, he considers that the stage of evolution which the great 

 elliptic nebula in Andromeda represents, is no longer a matter of 

 hypothesis. In a beautiful photograph of this nebula, taken by 

 Mr. Roberts, with an exposure of four hours, a planetary system 

 is shown at a somewhat advanced stage of evolution, and, to all 

 appearances, several planets have been already thrown off ; and 

 the central gaseous mass has condensed to a moderate size in 

 comparison with what it must have been before any planets 

 had been formed. 



Having now briefly noticed the advances made in our know- 

 ledge of the constitution of the stars, and of the almost infinite 

 numbers newly revealed to us by the photographic camera, there 

 is still another research to which I ought to allude, and which is 



