308 TRANSACTIONS OF SECTION A. 



dependent on this detail, but on the possession of the qualities above mentioned — 

 perception, faith, skill, and energy. 



By way of bringing out more fully the nature of the assertion made by 

 Sir George Darwin, let me beg your attention to a striking incident in recent 

 astronomical history. We all know how the great astronomer we lost last year, 

 Sir William Huggins (one of those already mentioned as having occupied the presi- 

 dential chair of the Association without having filled that of Section A), initiated 

 the determination of velocities of the heavenly bodies in the line of sight by 

 means of the spectroscope. We know further how the accuracy of these deter- 

 minations was improved by the application of photography, so that it has recently 

 become possible to measure the velocity of the earth in its orbit (as it alternately 

 approaches and recedes from a given star) with a precision which matches that 

 of other known methods. Now Mr. W. W. Campbell, on his appointment as 

 Director of the Lick Observatory in 1900, perceived the desirability of observ- 

 ing the line of sight velocities of as many stars as possible, believed that 

 that outcome would be in some way for the advancement of science, and resolutely 

 acted on that belief, so that for many years the resources of his great establish- 

 ment have been devoted to this work. He has not turned aside from it even to 

 publish provisional results, and has thereby incurred some adverse criticism. 

 But, having now accumulated a large mass of observation, he is proceeding to let 

 them tell their own tale, and a wonderful story it is. We have unfortunately 

 not time to listen to more than a fraction of it at the moment ; but that fraction 

 is well worthy of our attention. When the stars are grouped in classes according 

 to their spectral type, their average velocities differ; and if the spectral types are 

 arranged in that particular order which for quite independent reasons we believe 

 to be that of development of the stars, there is a steady increase in the velocities. 

 To put the matter in a nutshell, the older a star is the quicker it moves. There 

 are no doubt several assumptions made in reducing the matter to this simple 

 statement, but I venture to think that they do not affect the point I now wish to 

 make, which is as follows. There is no doubt whatever that the catalogue of facts 

 accumulated by Mr. Campbell, when arranged in an obvious order, has led to a 

 most important scientific generalisation — a direct negative at this date of Sir 

 George Darwin's opening sentence, however true it may have been when he wrote 

 it. If we read on, his next sentence doubtless entitles him to say that it was 

 the marshalling of the facts which led to the conclusion. It is not altogether 

 clear to me in what way this marshalling differs from the permitted ' arrange- 

 ment ' of the catalogue; but the third sentence seems to. imply that the distinction 

 lies in the existence of a theory. But certainly Mr. Campbell had no theory ; so 

 far is he from having had a theory that he finds it extremely difficult, if not at 

 present actually impossible, to formulate one, which will satisfactorily account 

 for the extraordinary fact brought to light by the simple arrangement of his 

 catalogue. 



Witness his words in Lick Observatory 'Bulletin,' No. 196, dated April 20 

 last :— 



The correct interpretation of the observed facts referred to in this 

 'Bulletin' seems not easy of accomplishment, and the brief comments which 

 follow make no pretensions to the status of a solution. 



That stellar velocities should be functions of spectral types is one of the 

 surprising results of recent studies in stellar motions, for we naturally think 

 of all matter as equally old gravitationally. Why should not the materials 

 composing a nebula or a Class B star have been acted upon as long and as 

 effectively as the materials in a Class M star ? . . . The established fact 

 of increasing stellar velocities with increasing agee suggest the questions : Are 

 stellar materials in the ante-stellar state subject to Newton's law of gravita- 

 tion ? Do these materials exist in forms so finely divided that repulsion under 

 radiation pressure more or less closely balances gravitational attraction ? 

 Does gravity become effective only after the processes of combination are 

 well under way ? 



Mr. Campbell is far from being helpless in the situation he has created ; he 

 is ready with suggestions, though he modestly puts them as questions: but they 

 are obviously consequent, and not antecedent, to the advance which he has made. 



