NA TURE 



[July 8, 1922 



from radium) on the short wave-length side as short 

 as o-o2 Angstrom units. Hence, the X-ray spectrum 

 now known comprises waves of all this great range of 

 wave-lengths. It w.ill also be remembered that other 

 researches, such as those of Lyman and Kurth, Mohler 

 and Foote, and Richardson and Bazzoni, have intro- 

 duced us to rays, termed the K, L, M, and N series, 

 derived by radiations from carbon, oxygen, iron, copper, 

 potassium, sodium, magnesium, and molybdenum, 

 which have wave-lengths ranging to 375 Angstrom 

 units, thus bridging over the gap between the shortest 

 ultra-violet ravs and X-rays. 



Prof. Niggli's contribution offers a survey of the 

 substances the crystal structure of which has now 

 been ascertained by the various X-ray methods of 

 Laue, the Braggs, Debye and Scherrer, and Hull, 

 including a table of the absolute dimensions of the 

 space-lattice cells resulting from the Bragg spectro- 

 metric measurements. His concluding remarks are 



well worth quoting (so far as is possible in a translation 

 from the German), especially when it is remembered 

 that Prof. Niggli has now taken over from Prof, von 

 Groth the editorship of the Zeitschrift fiir Krystallo- 

 graphie. " By Laue's discovery crystallography not 

 only obtains a new method of investigation, but 

 experiences a new ' liveliness ' in almost every one of 

 its branches. Most especially are we mineralogists 

 glad that our colleagues of the sister sciences now 

 bring to the crystal an entirely new attitude of mind 

 and interest than formerly, for only by the combined 

 and simultaneous labours of all can further research 

 move along right lines." 



With these words of Prof. Niggli we must all agree, 

 and it would appear that the sentiment is now so 

 universally accepted and recognised that the future 

 is bright with hope for a progress during the next 

 decade as glorious as that which is now recorded at 

 the termination of ten years of X-ray crystal analysis. 



Obit 



Prof. J. C. Kapteyn, For. ^Iem., R.S. 



T ACOBUS CORNELIUS KAPTEYN was born at 

 J Barneveld, Holland, on January 19, 185 1. He 

 studied at Utrecht from 1869 to 1875, and was then 

 appointed an observer at Leiden Observatory, where he 

 remained for two years. In 1878 he was appointed 

 professor of astronomy and theoretical mechanics at the 

 University of Groningen. He was in the unusual position 

 of an astronomical professor without an observatory, 

 and he immediately applied to the Dutch Government 

 lor the means to equip a students' observatory; he 

 mentioned in particular a 6-inch heliomet'er as desirable. 

 The application, however, was unsuccessful, and for 

 a few years his lectures monopolised his attention. 

 Then, finding that he had time to spare and no instru- 

 ments, he began to look about for some useful astronomi- 

 cal work of a computational kind that he could carry 

 out. Circumstances soon brought a task well fitted 

 to his tastes. 



Photography had been revolutionised by the intro- 

 duction of the gelatine dry plate about 1880, and its 

 astronomical possibilities were soon exemplified by 

 the successful photographs obtained of the comets of 

 1881 and 1882. In the latter case Sir David Gill 

 assisted the local photographers by letting them strap 

 their camera to an equatorial, with very successful 

 results ; he was impressed by the number of faint stars 

 that were visible on the plates, and the idea of a 

 southern photographic Durchmusterung quickly matured 

 in his mind. He found a willing collaborator in 

 Kapteyn, who volunteered to conduct the measure- 

 ments and reductions at Groningen. Funds were 

 collected from various sources ; the Government 

 Grant Committee of the Royal Society voted 300/. in 

 each of the years 1885 and 1886 ; this was, however, 

 stopped in 1887, it is believed from the notion that the 

 Astrographic Catalogue, which was then inaugurated, 

 would obviate the need for the Durchmusterung. If 

 that was the idea, subsequent events have proved it 

 to be incorrect. The Astrographic Catalogue is still 

 far from completion, while the Durchmusterung has 

 been available as a standard work for a quarter of a 



NO. 2749, VOL. I io] 



uary. 

 century. It might have been made more perfect but 

 for shortage of funds : the plates admitted measure- 

 ment to seconds of arc, but in practice this was limited 

 to tenths of minutes. Moreover, it was only carried 

 to declination 18 S., instead of to the equator. With 

 a view of shortening the reductions, Kapteyn devised 

 an ingenious measuring instrument, which was practic- 

 ally a small equatorial placed in the position, relatively 

 to the plate, occupied by the centre of the camera lens, 

 the principle being that, since the rays through this 

 point suffered no bending, the star-images, viewed 

 from here, have the same configuration as the stars 

 themselves. Hence right ascension and declination 

 could be read from the circles. 



The whole work occupied thirteen years, nearly 

 double the original estimate, but the time was spent 

 ungrudgingly by Kapteyn, and the close examination 

 and discussion of the results brought to light many 

 interesting facts, such as the change of colour-index 

 with galactic latitude, the galactic stars being bluer 

 than the non-galactic ones. It was also found In- 

 careful counts that there was no sensible difference 

 in the number of stars recorded at the centres of the 

 plates and near their edges. Several cases of light 

 variation and of rapid proper motion were also found. 

 The question of photographic stellar magnitudes was 

 still in its infancy, but a simple formula was found, 

 mag. = B/(C + diam.), B and C being constants for the 

 plate ; as these are printed, it is possible to recover 

 the diameter of each star. 



Kapteyn was elected an associate of the Royal 

 Astronomical Society in 1892, and received its Gold 

 Medal in 1902 in appreciation of his work on the 

 Durchmusterung. This was, however, only one of the 

 numerous researches that he undertook to investigate 

 the structure of the sidereal universe. He saw the 

 need for increased knowledge of stellar parallaxes. 

 In 1886 he investigated the parallaxes of forty-five 

 stars by the method of meridian transits (since found 

 to be less accurate than the photographic method), 

 and endeavoured to secure that the astrographic 

 plates should each have three exposures at dates of 

 maximum parallactic displacement. This was not 



