182 



SIR DAVID GILL, ON THE SIDEREAL UNIVERSE. 



pictures of areas of it about 15° square, taken with a lens of 

 short focus, in which the star images are necessarily very close 

 together. These pictures, taken by Mr. Barnard, illustrate in a 

 striking manner the grouping of the thousands of faint stars, 

 clusters and nebulae which together make up the light of 

 the Milky Way when the latter is viewed without optical aid. 

 (Here the lecturer exhibited photographs of — 



The great star cloud in Sagittarius. 

 The small stai* cloud in Sagittarius. 

 The region about 0 Ophiuci. 



Another region in Ophiucus, with nebula near p Ophiuci.) 

 Now these nebulai — what are they ? 



Some are certainly gaseous, as proved by their spectra. The 

 spectra of some nebulae, as first proved by Sir William Huggins, 

 instead of showing continuous spectra crossed by dark lines as 

 in the spectra of the sun and starS; are made up simply of 

 bright lines, which indicate a gaseous constitution. We find 

 many stars — notably the stars of the Pleiades — enveloped in 

 such nebulae. 



Apparently these stars have been formed out of this nebulous 

 stuff that surrounds them, in accordance with the nebular 

 hypothesis of Laplace. 



Examples of gaseous nebulae were shown on the screen, 

 such as — 



The Pleiades after different lengths of exposure. 



The great nebula in Orion. 



Nebula about 7} Argus 



The Crab nebula. 



The ring nebula in Lyra. 



The planetary nebula 37 H IV Draconis. 



This last was the nebula which first revealed to Sir William 

 Huggins the gaseous constitution of some nebula. The beautiful 

 Mount Wilson photograph shows this nebula to have a helix-like 

 structure. ' 



We have some spiral nebulae such as M 64, Comae Bernicis 

 (shown on the screen), which also seem to be purely gaseous; 

 but there are others like — 



The great nebula in Andromedal 



M 33 Trianguli 

 M 81 Ursae Majoris 

 Spiral nebula in Can. Ven. 



which have continuous or stellar-like spectra. 



