SECTIONAL TRANSACTIONS.—A. 445 
reservation, however, that it is not ‘cosmic’ time that enters into the 
formule, but the peculiar time of this observer. In other words, the so- 
called ‘ cosmic ’ time of an event is nothing more than the time an observer 
chooses to assign to it. It happens that in Lemaitre’s simplified theory the 
observers are all so alike that their ‘ times ’ can be co-ordinated so as to give 
the impression of a single ‘ cosmic’ or ‘ absolute ’ time in the universe. 
In conclusion, reference should be made to a question which may have 
occurred to most of you. If our universe is flying to pieces at the present 
astounding rate, how is it that periodic systems in the universe such as our 
Sun and his planets seem so excessively stable and permanent? The 
answer is supplied by the theory of condensations, which shows that an 
observer using the methods of measurements terrestrial astronomers employ, 
necessarily concludes that planetary systems are fixed and unchanging in 
size, whilst the system of the nebule expands. Conversely, if we imagine 
a cosmical being who looked upon the system of the nebule as fixed and 
unchanging, this being would see the Sun and his planets shrinking away to 
nothing. Indeed, the nature of our minds is such that we instinctively 
endow our immediate surroundings with an element of permanency, and 
relegate to the distant nebulz the evidences of the instability of the world 
in which we live. 
Dr. W. H. McCrea.—The relation of Milne’s theory to general 
relativity. 
I wish to say something about the relation of Prof. Milne’s theory of 
world structure to the general relativity theory of the expanding universe. 
There is a temptation to start with a moralising homily on the history of 
the subject. General relativity, through the work of Prof. de Sitter, 
actually predicted the systematic recession of distant nebulez. Data derived 
subsequently by Hubble and others gradually accumulated to provide 
observational support for this apparent recession. Result: one up for 
general relativity! The next development was that Friedmann, Lemaitre 
and others investigated non-static universes and arrived at the concept of 
the expanding universe—still using general relativity theory. The result 
was to make a systematic recession, or at any rate a systematic motion, of 
distant nebulze seem inevitable. At the same time an explanation was 
given of the particular form of Hubble’s empirical law for the observed 
variation of velocity of recession with distance. I should say, however, that 
the inevitableness of the phenomenon was apparent only to the initiated. 
For it was all very sophisticated and bound up with the difficult ideas of 
the curvature of space-time, and the mysterious cosmical constant A. 
In passing one might comment on the prevalent fashion of giving \ a name. 
Sir Arthur Eddington himself calls it ‘ Gulliver.’ But I fancy he ought to 
call it ‘Mrs. Harris.’ For this friend of Sir Arthur’s gets the credit for 
marvellous doings—as, in fact, he has just been telling us. Nevertheless, 
does she really exist? Einstein and de Sitter have been irreverent enough 
to suggest that she might be a zero quantity ! 
To pursue our history, however, we next come to an unexpected move. 
Prof. Milne suggested an explanation of the scattering of the galaxies, which 
he has just explained to us, which makes the whole thing seem intuitive in 
an absurdly simple way. As Milne himself has remarked, had this explana- 
tion occurred to anyone before the advent of general relativity, it would 
probably have been immediately hailed as the obvious one. At this point 
someone is doubtless heard to murmur: ‘ What a pity Prof. Milne did not 
arrive on the scene fifteen years sooner and save us all this trouble of trying 
