326 



NATURE 



[January 5, 191 1 



The discrepancies in these quantities found by different 

 investigators, either starting with different data or 

 utilising different methods for the combination and dis- 

 cussion of the same material, had long been a puzzle to 

 astronomers. The key to the situation was at length 

 furnished by Prof. Kapteyn, of Groningen, who, in an 

 epoch-making paper read before the British Association in 

 Cape Town, first pointed cut that the apparent motions 

 of the stars indicated, not merely the existence of a 

 single solar apex, but that there were two separate regions 

 of the sky towards which a preference was shown by the 

 directions of motion of the Bradley stars. 



This was a phenomenon which could not be explained 

 by a simple motion of translation of the sun, as evidently 

 the sun's motion could not be directed to two different 

 points simultaneously, and the only feasible explanation 

 was that the stars consisted of two groups, and that the 

 motion of the sun relatively to one of these groups differed 

 from its motion relatively to the other, or that, though 

 the stars appeared intermingled in space, they possessed 

 an independent relative motion, which might be regarded 

 as located in one group or in the other, but which was 

 shared by all the stars peculiar to the group. 



The theory of the existence of two streams or drifts 

 of stars thus put forward by Kapteyn has since received 

 full confirmation by other investigators, notably by 

 Eddington, who based his examination on the early 

 observations of Groombridge, and by Dyson, who limited 

 his discussion to a selected list of stars possessing con- 

 siderable proper motions. 



Recent investigations at the Cape have led us to ex- 

 amine in somewhat more minute detail the proper motions 

 of the Bradley stars, with the result that, though the 

 phenomena first noticed by Kapteyn stand out as the most 

 prominent feature, certain subsidiary features of no less 

 importance have been brought to light. 



I have concerned myself hitherto only with the visible 

 motions of the stars transverse to the line of sight, as 

 derived by the older methods of measurement. The intro- 

 duction of the spectroscope into astronomical research has 

 opened up vast new fields into which, so far as they relate 

 to the chemical and physical constitution of the sun and 

 stars, it is not my purpose to enter to-night. What I 

 wish rather to emphasise is the value of this instrument 

 as a supplement to the older methods in relation to the 

 geometrical astronomy of position. 



In accordance with the principle laid down by Doppler, 

 the wave-length of light received from a source which is 

 either receding from or approaching a receiver will appear 

 to be modified by an amount dependent in a known 

 manner on the velocity of approach or recession. If the 

 receiver takes the form of a spectroscope which permits 

 by any means, direct or indirect, of the measurement of 

 the wave-lengths, and the normal wave-lengths of the 

 lines under examination are independently determined by 

 laboratory investigations, the difference between the 

 observed and the normal wave-lengths will thus afford a 

 means of measuring the velocity of approach or recession 

 of the source of light. 



Of the precautions necessary to ensure precision it is 

 not my purpose to speak to-night. The large spectro- 

 scope of the Cape Observatory, which we owe to the 

 munificence of the late Mr. Frank McClean, was from the 

 outset constructed with due regard to these precautions, 

 so far as they could be foreseen, for the purpose of deter- 

 mining with the greatest accuracy attainable the radial 

 velocities of stars. The instrument has been already 

 successfully used, and its capabilities have been estab- 

 lished in an investigation of the aberration constant of 

 light as depending on the apparent variations in the radial 

 velocities of stars resulting from the earth's orbital motion. 



From a relatively short series of observations discussed 

 by my colleague Dr. Halm, this constant has been derived 

 with a precision not inferior to that attained by the best 

 series of older observations, and the capabilities of the 

 method are yet far from exhausted. 



-At the present time the instrument is being devoted to 

 a series of observations of all such stars as are accessible 

 in the southern skies, the spectra of which present 

 sufficiently pronounced features to admit of measurement, 

 primarily with the view of ascertaining what evidence can 



NO. 2149, VOL. 85] 



be derived from a study of the radial velocities in regard 

 to the systematic structure of the universe. 



A year or two must elapse before the present observing 

 programme is completed, but a preliminary discussion of 

 the observations already secured in combination with the 

 published results derived from similar observations in the 

 northern hemisphere has revealed the existence of 

 anomalies similar to those found from the study of the 

 transverse motions — anomalies which can only be recon- 

 ciled with the two-drift hypothesis put forward by 

 Kapteyn by the further hypothesis that though both drifi- 

 pervade the whole sky, they are not similarly distribute 

 throughout it. 



At present, through scantiness of material, from a study 

 of the radial velocities we have been able to do little more 

 than discriminate between the two halves of the sk' 

 which contain, respectively, the greatest and the le.i 

 proportion of second drift stars. It is, however, a fact ot 

 some significance that the former corresponds very closely 

 with that hemisphere which contains the Milky Way, 

 suggesting the phenomenon that Kapteyn 's second drift 

 might be identified with the galaxy. It was with the view 

 of examining this suggestion in the light of the evidence 

 which could be secured from the transverse motions of the 

 Bradley stars that the discussions I have sketched to you 

 to-night were undertaken by Dr. Halm. 



While they have established almost beyond question the 

 rough features of distribution demanded to reconcile the 

 radial-velocity determinations, they further point to an 

 even more detailed correspondence between the distribu- 

 tion of galactic stars and the distribution of stars of the 

 second drift, leaving but little doubt as to the identity of 

 this second drift with the galaxy. It is this second drift 

 which exhibits evidence of structural unity. .As regards 

 the Milky Way, the mere appearance on any fine night 

 affords evidence of a similar character, and it is on this 

 account that we have been able to identify the Milky Way 

 with the second drift rather than with the first. 



The significance and origin of this structure are as yet 

 obscure, but the more its details are elucidated and the 

 essential features established the nearer are we to an 

 answer to the question. What is the Milky Way? 



To revert to my original text, I have endeavoured to 

 point out to you the method<v of research by which an 

 answer is sought to this and similar questions, and to 

 explain to you the reasons why the highest precision 

 attainable is a sine quA non in the conduct of such 

 research. Thus it is that the study of the large-scale 

 phenomena of the universe resolves itself frequently into 

 a study of the minute detail of instrumental appliances, 

 on which must be brought to bear all the knowledge that 

 can be derived from other branches of scientific work. 

 The geologist helps us in the selection of stable founda- 

 tions on which the engineer may erect our large instru- 

 ments. Chemistry and physics in our photography, our 

 optical and electrical appliances, are of daily application, 

 while one of the most valued accessories in almost all 

 methods of precise measurement is the spider's web we 

 derive from zoology. 



Astronomy, in its turn, has done much in the past, and 

 in the future will doubtless do more, to assist the develop- 

 ment of collateral sciences. Thus the geologist cannot 

 afford to ignore, even if he does not accept as conclusive, 

 the evidence furnished by astronomy as to the nature of 

 the earth's crust. Exact measurements of space and time 

 as conducted in physical laboratories are for the most part 

 conducted by methods first designed to suit the require- 

 ments of astronomical precision, while in the sun and 

 stars chemical phenomena, which may be studied with the 

 aid of the spectroscope, are taking place on a scale far 

 surpassing anything that can be produced in the labora- 

 tory. 



The value gf free intercourse between workers in the 

 various branches of science is certainly indisputable, and 

 I wish to close my address by reference to the opportuni- 

 ties which our society can afford in this respect.^ Devotees 

 even of applied, and still more of pure, science in a young 

 country are necessarily few in number and scattered. A 

 large proportion of these will in the early stages of their 

 career have been in close association with one or the other 

 great centres of scientific activity of the world, and to 

 such a feeling of scientific isolation almost amounting to 



