October 22, 1891 



NATURE 



605 



The observer's work during the exposure is simply to direct 

 the telescope to the required part of the sky, and then the clock- 

 work nearly does the rest— but not quite so. The observer 

 holds in his hand a little electrical switch with two keys; by 

 pressing one key he can accelerate the veloci ty of the driving- 

 screw by about i per cent., and by pressing the other he can 

 retard it i per cent. In this way he keeps one of the stars in 

 the field always perfectly b.isected by the cross-wires of his 

 guiding telescope, and thus corrects the small errors produced 

 partly by changes of refraction, partly by minute unavoidable 

 errors in cutting the teeth of the arc into which the screw of the 

 driving-shaft of the clockwork gears. 



The work is monotonous rather than fatiguing, and the com- 

 panionship of a pipe or cigar is very helpful during long ex- 

 posures, A man can go on for a watch of four or five hours 

 very well, taking plate after plate, exposing each, it may be, 

 forty minutes or an hour. If the night is fine, a second observer 

 follows the first, and so the work goes on the greater part of the 

 night. Next day he develops his plate, and gels something like 

 this. [Star-cluster.] 



Working just in this way, but with the more humble appara- 

 tus which you see imperfectly in the picture now on the screen, 

 we have photographed at the Cape during the past six years the 

 whole of the southern hemisphere from 20° of south declination 

 to the South Pole. 



The plates are being measured by Prof. Kapteyn, of Gronin- 

 gen, and I expect that in the course of a year the whole work, 

 containing all the stars to 9^ magnitude (between 200,000 and 

 300,000 slars) in that region, will be ready for publication. 

 This work is essential as a preliminary step for the execution in 

 the southern hemisphere of the great work inaugurated by the 

 Astrophotographic Congress at Paris in 1887, the last details of 

 which were settled at our meeting at Paris in April last. What 

 we shall do with the new apparatus, perhaps I may have the 

 honour to describe to you some years hence, after the work has 

 been done. 



We now come to an important class of astronomical work, 

 more purely astrophysical, for the illustration of which I can no 

 longer appeal to the Cape, because I regret to say that we are 

 not yet provided with the means for its prosecution. I refer to 

 the use of the spectroscope in astronomy, and especially to the 

 latest developments of its use for the accurate measurement of 

 the velocity of the motions of stars in the line of sight. ^ 



It is beyond the province of this lecture to enter into history, 

 but it is impossible not to refer to the fact that the chief im- 

 pulse to astronomical work in this direction was given by Dr. 

 Huggins, our Chairman tonight— nay, more, except for the 

 early contributions of Fraunhofer to the subject, Dr. Huggins 

 certainly is the father of sidereal spectroscopy, and that not in 

 one but in every branch of it. He has devised the means, 

 pointed the way, and, whilst in many branches of the work he 

 still continues to lead the way, he has of necessity left the 

 development of othtr branches to other hands. 



From an astronomer's point of view the most important ad- 

 vance that has been made in spectroscopy of recent years is the 

 sudden development of precision in the measures of star motion 

 in the line of sight. The method remained for fifteen or sixteen 

 years quite undeveloped from the condition in which it left the 

 hands of Dr. Huggins, and certainly no progress in the accuracy 

 attained by Dr. Huggins was made till the matter was taken up 

 Dr. Vogel at Potsdam. At a single step Dr. Vogel has raised 

 the precision of the work fiom that of observations in the days 

 of Ptolemy to that of the days of Bradley— from the days of 

 the old sights and pinnules to the days of telescopes. There- 

 fore I take a Potsdam observation as the best type of a modern 

 spectroscopic observation for description, especially as I have 

 recently visited Dr. Vogel at Potsdam, and he has kindly given 

 me a photograph of his spectroscope, as well as of some of the 

 work done with it. 



A photograph of the Potsdam spectroscope attached to the 

 equatorial is now on the screen. [Description.] 



The method of observation consists simply in inserting a small 

 photographic plate in the dark slide, directing the telescope to 

 the star, and keeping the image of the star continuously on the 

 slit during an exposure of about an hour ; and this is what is 

 obtained on development of the picture. 



If the star remained perfectly at rest between the jaws of the 



' The older methods enabled us to measure motions at right angles to the 

 line of sight, but till the spectrcscope came we could not measure motions in 

 the line of sight. 



NO. 



I 147, VOL. 44] 



slit the spectrum would be represented by a single thread of 

 light, and of course no lines would be visible upon such a 

 thread ; but the observer intentionally causes ihe star image to 

 travel a little along the slit during the time of exposure, and so a 

 spectium of sensible width is obtained. 



You will remark how beautifully sharp are the faint lines in 

 this spectium. Those who have tried to observe the spectrum 

 of Sirius in the ordinary way, know that many of these fine lines 

 cannot be seen or measured with certainty. The reason is that 

 on account of irregularities in atmospheric :efraction, the image 

 of a star in the telescope is rarely tranquil, sometimes it shines 

 brightly in the centre of the slit, sometimes barely in the slit at 

 all, and the eye becomes puzzled and confused. But the photo- 

 graphic eye is not in the least disturbed ; when the star image is 

 in the slit, the plate goes on recording what it sees, and when 

 the star is not in the slit the plate does nothing, and it is of no 

 consequence whatever how rapidly these alternate appearances 

 and disappearances recur. The only difference is that when 

 the air is very steady and the star's image, therefore, always in 

 the slit, the exposure takes less time than when the star is 

 unsteady. 



That is one reason why the Potsdam results are so accurate. 

 And there are many other reasons besides, into which I cannot 

 now enter. What, however, it is very important to note is this, 

 that we have here a method which is to a great extent inde- 

 pendent of the atmospheric disturbances which in all other 

 departments of astronomical observation have imposed a limit 

 to their precision. Accurate astrospectroscopy, therefore, may 

 be pushed to a degree of perfection which is limited only by the 

 optical aid at our disposal and by the sensibility of our photo- 

 graphic plates. 



And now I think we have sufficiently considered the ordinary 

 processes of astronomical observation to illustrate the character 

 of the work of an astronomer at night. The picture should be 

 completed by an account of his work by day ; but to go into 

 that matter in detail would certainly not be within the limits of 

 this lecture. It is belter that I .should in conclusion touch upon 

 some recent remarkable results of these day and night labours. 

 It is these after all that most appeal to you ; it is for these that 

 the astronomer labours ; it is the prospect of them that lightens 

 the long watches of the night and gives life to the otherwise 

 dead bones of mechanical routine. 



Let us take first some spectroscopic results. To explain their 

 meaning let me remind you for a moment of the familiar analogy 

 between light and sound. 



The pilch of a musical note depends on the rapidity of the 

 vibrations communicated to the air by the reed or string of the 

 musical instrument that produces the note, a low note being 

 given by slow vibi^ations and a high one by quick vibrations. 



Just in the same way red light depends on relatively slow 

 vibrations of ether, and blue or violet light on relatively quick 

 vibrations. Well, if there is a railway train rapidly approaching 

 one, and the engine sounds its whistle, moie waves of sound 

 from that whistle will reach the ear in a second of time than 

 would reach the ear were the train at rest. On the other hand, 

 if the train is travelling at the same rate away from the observer, 

 fewer waves of sound will reach his ears in a second of time. 

 Therefore an observer beside the line should observe a distinct 

 change of pitch in the note of the engine whistle as the train 

 passes him, and as a matter of fact such a change of pitch can 

 be and has been observed. 



Just in the same way, if a source of light could be moved 

 rapidly enough towards an observer it would become bluer, or 

 if away from him it wou'.d become more red in colour. Only it 

 would require a change of velocity in the moving light of some 

 thousands of miles per second in order to render the difference 

 of colour sensible to the eye. The experiment is, therefore, not 

 likely to be frequently shown at this lecture table ! 



But the spectroscope enables such changes of colour to be 

 measured with extreme precision. Here on the screen is the 

 most splendid illustration of this that exists at present, viz. 

 copies of three negatives of the spectrum of o Aurigae, taken at 

 Potsdam in October and December of 1888, and in March 

 1889. 



The black line (the picture being a negative) represents the 

 bright line H7 given by the artificial light of hydrogen, the 

 strong white line in the picture corresponds to the black absorp- 

 tion line which is due to hydrogen in the atmosphere of the 

 star. 



Why is it that the artificial hydrogen line does not correspond 



