316 Chronicles of Science. [April 



IX. PHYSICS. 



Light. — Light and astronomy are so nearly allied, that it is frequently 

 a matter of difficulty to know how to apportion the subjects treated 

 of in these Chronicles. This difficulty is more especially felt at the pre- 

 sent time, when spectrum-analysis — a siibject which has been hitherto 

 hovering between chemistry and optics — has suddenly shot off to 

 the sun and fixed stars, and is commencing more distant flights to the 

 nebulae. We have divided the subjects in this branch as well as the 

 circumstances will admit ; but the natural tendency which there has 

 been of late years to the mutual interpenetration of all the sciences, 

 will necessarily cause a great deal of chemistry and astronomy to 

 come into optics, as well as astronomy and optics into chemistry. 

 With this explanation, we will proceed to chronicle a most important 

 service which optical science has done for her great sister astronomy 

 — a service the more remarkable, inasmuch as astronomy, whilst she 

 was really unable to solve the difficult question, fancied that the 

 whole subject was a very straightforward one, and had fallen into an 

 error which it will take perhaps the remainder of this century to 

 eradicate from astronomical text-books. We refer to the constitution 

 of the nebulae. These bodies are generally divided into resolvable 

 and irresolvable, the former being split up under more or less power- 

 ful telescopic means into bright points, which have been always con- 

 sidered stars, whilst the irresolvable ones have been generally regarded 

 as being similar clusters of stars, too far off, however, to be resolved 

 into discrete points by our present optical means. As the aperture 

 and defining power of telescopes increased, nebulae of the latter class 

 have one by one been transferred to the former, until the opinion has 

 gradually prevailed, that all nebulae would ultimately prove to be 

 resolvable. Although serious doubts as to the generality of such a 

 law have been entertained by many eminent astronomers, the question 

 would have long remained unsolved had not the services of spectrum- 

 analysis been called into requisition. 



In a paper recently presented to the Eoyal Society, Mr. William 

 Huggins gave the results of the application of prismatic analysis to 

 some of the objects in the heavens known as nebulas. Eight of the 

 nebulae examined gave a spectrum indicating gaseity, and of these six 

 belong to the class of small and comparatively bright objects, which 

 it is convenient to distinguish still by the name of planetary. These 

 nebulae present little indication of probable resolvability, even with 

 the greatest optical power which has yet been brought to bear upon 

 them. The other two nebulae, which gave a spectrum indicative of 

 matter in the gaseous form, were the annular nebida in Lyra and the 

 Dumb-bell nebula. The results of the examination of these nebulae 

 with telescopes of great power must probably be regarded as in 

 favour of their consisting of clustering stars ; it was therefore of 

 importance to determine by the observation of other objects, whether 

 any nebulae which have been certainly resolved give a spectrum which 

 shows the source of light to be glowing gas. With this purpose in 



