42 



TRESIDEXTIAL ADDRESS SECTIOX A. 



The enormous distances deduced for the globular clusters 

 IS very much exceeded by those found for the spiral nebula? 

 by these indirect methods. The spectroscope has shown that 

 the spiral nebulae have radial velocities very much greater 

 than those of the stars. This fact, combined with the absence 

 of any detectable cross or proper motion indicates their 

 distance to be very much greater than the stars. The 

 discovery of novae or new stars within certain of these 

 nebulae confirms this idea, and has led to estimates of their 

 distance. The theory is, that whatever celestial catastrophe 

 it is which gives rise to the outburst of a new star, the order 

 of magnitude is the same, whether the occurrence takes place 

 within a spiral nebula or within our own Milky Way. Hence, 

 from the apparent brightness of the new star in the nebula, 

 its distance can be estimated in terms of the distance of the 

 Milky Way. 



Such ideas as these concerning- the existence of huge 

 stellar systems at inconceivably great distances, or " island 

 universes," are only tentatively accepted by astronomers. 

 They represent logical deductions from observed facts, but 

 the facts are still very meagre. Also, they involve extra- 

 polations from physical laws based upon experience of the 

 behaviour of matter under the conditions existing on this 

 Earth, and the curve of our experience may in its remoter 

 branches take some unforeseen direction. For instance, 

 certain facts connected with the Great Nebula in Orion can 

 only be explained by supposing that the mass of gas which 

 constitutes the nebula must have a density less than 

 one-millionth of that of ordinary air at sea-level, and yet in 

 some way or other this gas is incandescent. It must be 

 admitted that our knoAvledge of matter in this state is very 

 meagre. 



Thus these ideas can only be accepted with caution until 

 the convergence of different lines of research approaching 

 from different directions and the steady accumulation of 

 unassailable facts puts them on a firmer foundation. If they 

 are wrong, they will fall to the groimd, but even a false 

 hypothesis has served a useful purpose in indicating methods 

 of research. In this region of astronomy the astronomer 

 waits somewhat upon the physicist and chemist to inform 

 him of the behaviour of matter under very varied conditions 

 of pressure and temperature. 



