496 



NATURE 



[March 26, 1896 



THE MEASUREMENT OF DOUBLE-STARS 

 BY INTERFERENCE. 



AN interesting form of micrometer is described by Herr Karl 

 Schwarzschild in Astroiioinischen Nachrichten, No. 3335. 

 The idea is gathered from the instrument which Michelson sug- 

 gested and used for measuring small diameters and distances, an 

 account of which appeared in the Memoirs of the National 

 Academy of Science, Washington, 1891. Michelson, it may be 

 remembered, placed before the object-glass of his refractor a 

 disc in which were two parallel movable slits that set up inter- 

 ference phenomena ; and an observation consisted in noting 

 simply the disappearance and reappearance of the interference 

 bands. Schwarzschild's disc, or more accurately oblong frame- 

 work, on the other hand, contains several slits cut out at equal 

 distances from one another, which cause several images to be 

 visible at the eye-end of the telescope, forming a true multiple- 

 image micrometer. 



If one considers the case of the ordinary glass grating as used 

 in spectroscopic work, it is well known that with a bright point 

 as the source of light, we obtain a series of images, the angular 

 distances of which from the unrefracted central image, for a 

 certain wave-length, are given by the formula sin a = A.w/a', 

 when A is the wave-length, d the distance between the lines on 

 the grating, and n their number. In the case of daylight the 

 centre image becomes sharp and white, while the others become 

 broader and broader, in fact small spectra. By exaggerating 

 this idea of the grating, and cutting out of a card slits three milli- 

 metres broad and ten millimetres distant from one another, the 

 angular distances of the images for wave-length A = 570 /^/i be- 

 come very small, and can hardly be separated with the unaided 

 eye. Such a grating as this placed before the object-glass of a 

 telescope directed to a star would show, in the iield of view, one 

 colourless image accompanied on its right and left sides by several 

 other images, the first of which would differ only slightly in 

 sharpness and colour from the middle image. It is only to these 

 three images that Schwarzschild pays attention. Of course it 

 is necessary that some means should be at hand by which these 

 images may be moved with respect to one another, and this he 

 accomplishes very simply. 



To the object-glass ring, and in a plane parallel to it, he fixes 

 a framework capable of rotation in this plane. Two circular 

 rods, at opposite ends of a diameter of the object-glass, and 

 perpendicular to the framework, are rigidly fixed to the 

 latter, and to these rods is connected the apex of two 

 inclined smaller frameworks containing the slits, the other two 

 extremities of which slide in the grooves of the large frame. 

 Since the distance of this apex from the object-glass can, by 

 means of a rack and pinion movement, be increased or decreased, 

 and since also the distances of the different slits vary consequently 

 in a simple known manner, the displacement of the images in 

 the field of view can be easily calculated. 



In bringing a double star into the field of view, two parallel 

 series of images would thus be seen, one series from the primary, 

 the other from the companion. The whole framework contain- 

 ing the grating was then rotated in position-angle until the two 

 lines of images coincided ; the position of this line was then 

 determined by means of a micrometer eyepiece, and the position- 

 angle read off. To measure the distance between the stars, the 

 rack and pinion motion connected with the apex of the two 

 frames containing the grating was then used, until the image of 

 he companion appeared exactly between the two images of the 

 primary. The position of the grating was then read oft", and a 

 brief calculation gave the distance required. P'rom a series of 

 observations of several binaries, the total mean gave as a probable 

 error of the mean for each evening : 



Probable error 



Distance in Distance in Position angle 



2""3 o"-o5o o"-oS2 



The numbers show, as Herr Schwarzschild points out, that 

 greater accuracy can be obtained by this means than by the 

 thread micrometer. He is not, however, very confident about 

 the usefulness of the method, for when the distances to be 

 measured exceed 5", the colour of the first images becomes very 

 apparent, and thus destroys the accuracy in measuring. Further, 

 the nature of the method stops it from being useful for measur- 

 ing pairs dimmer than the 7th magnitude, because the aperture 

 of the object-glass is cut down very considerably when the 

 grating is much inclined and the light is distributed over several 



NO. 1375, VOL. 53] 



images. The simplicity of the method has, however, much to- 

 recommend itself to many, more especially to those who possess 

 large apertures, and can therefore afford to spare a little light. 



D. 



FLORA OF ZERAFSHAN. 

 TN a communication to the St. Petersburg Society of Naturalist 

 -*- {Proceedings, 1895, i-). M. Komaroff thus sketches the 

 flora of the Zerafshan region of Russian Turkestan. The Aral- 

 Caspian flora covers the lowlands up to the 1000 to 1500 feet 

 level. Next comes the prairie, or Steppe, zone, which spreads 

 up to about 3500 feet of altitude. In its upper parts it is 

 characterised by Steppe-bushes, of which the almond-tree 

 {Amygdalus spittosissivitis, Bunge) is the most characteristic 

 representative. Higher up, from 3500 to 6000, or 6200 feet, 

 comes the zone of deciduous trees, which may be subdivided 

 into a lower sub-zone of Mediterranean trees {Pistacia vera^ 

 Celtis australis, Amygdalus communis, Acer motispesstilantim , 

 &c.), with a prevalence of fragrant Labiatae, which attains 

 approximately the 4500 feet level ; and an upper sub-zone 

 characterised by maple-trees (^(-^r /ac///w). The zone between 

 6000 feet and nearly 8500 feet is taken by the Juniperus-trees 

 which correspond in Zerafshan to the coniferous trees of other 

 regions. It is covered at its upper limits with rampant bushes 

 of Jnniferiis nana and pseudosabina, Comariim, Cotoneaster, 

 Lonicera, Astragalus, and so on. The Alpine zone attains the 

 levels of 11,000 and 12,000 feet — the morainic plants, 

 Didymophysa fedtschenkoana, Corydalis fedtschenkoana, Ceras- 

 tium lithospermifoliiim, Saxifraga axillaris and Allardia 

 tomentosa reaching the highest altitudes. On the Zerafshan 

 glacier, at a spot where it was covered with some gravel, the 

 author found specimens of Saxifraga axillaris, Epilobium 

 latifoliwn, Arabis tibetica, Poa karatavica, and one Carex — a 

 fact which shows how careful one must be in interpreting the 

 real sense of plant-bearing strata imbedded amidst morainic 

 deposits. It is also worthy of note, that the botanic zones 

 of Zerafshan very much correspond to the zonal geological 

 structure of the highlands. The Aral-Caspian flora covers the 

 seolic deposits of the great desiccated inner sea of Central Asia ; 

 the Steppe flora covers the Loess girdle ; the Mediterranean 

 trees and shrubs occupy the limestones and the marls, while 

 the Juniperus zone spreads over the crystalline slates and lime- 

 stones, and the Alpine flora covers the higher granitic massives 

 of the highlands. Man evidently alters to a great extent the 

 character of the vegetation — pistachio-tree groves and the 

 Juniperns excelsa trees being rapidly destroyed ; while the 

 hundreds of thousands of sheep which are brought every year to 

 the Zerafshan mountains from the lowlands, entirely destroy the 

 Alpine prairies — thickets of Artemisia dracunciilns taking the 

 place of the grasses. 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE. 

 Oxford. — One of the arguments which are brought forward 

 for granting the degree of B.A. to students from Newnham and 

 Girton Colleges is that, when they apply for educational posi- 

 tions, they are at a disadvantage as compared with students 

 from other universities which do grant the degree of B.A. Dr. 

 W. H. Besant points out in a circular, which he had addressed 

 to the members of the Senate, that this difficulty would be en- 

 tirely removed if a charter were granted to Newnham and Girton 

 Colleges, creating a Women's University, which should have 

 the power of granting degrees. This need not interfere with 

 the present arrangements for the teaching and for the examina- 

 tion of the students in the various subjects, the study of which 

 they now undertake. Mr. J. L. Strachan-Davidson has sug- 

 gested that Oxford, Cambridge, and Dublin — the three univer- 

 sities which have not as yet conferred degrees on women — 

 should join in a petition to the Crown to grant a charter for a 

 university whose sole function it shall be to give degrees to 

 women. A scheme similar to that supported by Dr. Besant has 

 been practically adopted by the University of Harvard. 



The members of the Skinners' Company visited Tonbridgc 

 on Saturday last, for the purpose of opening a new second-grade 

 school which they have just built at a cost of upwards of 



;,^I0,000 



