November 30, 1893] 



NATURE 



III 



OUR ASTRONOMICAL COLUMN. 



Otto Struve's Double-Star Measures. — The most im- 

 portant addition to double-star astronomy during the last year is 

 without doubt the work which we owe to OUo Struve, and 

 which is entitled "Mesures Micrometriquesdes Etoiles Doubles" 

 {Observations de Poulkova Tome IX. (avec un supplement) et 

 Tome X.). The period which the observations cover is very 

 large when one considers that it is for one observer, commencing 

 as it does with the observations made in the year 1837, when 

 Otto Struve was only seventeen years old. Readers who are 

 unable to approach these volumes themselves will find that M. 

 Bigourdan, in the October number of the Bulletin Astronomique, 

 gives a general summary of the whole of the contents. As one 

 would expect, the introductions to the volumes contain a 

 mine of important information, both with regard to the measures 

 and to the puzzling question of the " personal equation," a 

 question on which even to-day astronomers hold different views. 

 Otto Struve busied himself especially in this direction, making, in 

 the years 1853-1876, a series of measures of artificial double stars. 

 The expressions for the corrections which he obtained assumed 

 considerable proportions, as will be seen below, the first being 

 that for angles of position, and the second that for distance : — 



Position angle 



5" "2 , 4"-4sin(2()> - 27° 13') 



Corr. = -I- 



I 4- 0'20^ 



I -h 0-14(3 "3 - gf 

 5°-6sin(4(^ - 25° o') 



Distance 

 Corr. = 



6'-oif>{g - 2'o) 



4- 



I -^ o'2o^- 



o"'i5 cos(2<^ - 28 '4) 



I -f o-09(4-2 - gf I + o 06(5 -2 - gf 



when^ represents " I'angle visuel du couple considere experime 

 €n prenant pour unite celui qui correspond au grossessement de 

 708 fois," and ^ is the angle of the line between the two stars 

 and the vertical. 



Whether such corrections as these, made under non-observa- 

 tional conditions, should be applied to measures actually made 

 in the sky is still open to much doubt. Otto Struve discusses 

 also the observations made at Pulkova with those made at the 

 same epoch by different observers ; the comparison, to take an 

 •example, shows that Dawes's position angles in his early mea- 

 sures appear free from systematic error, while those made later 

 require a correction of -f i^'8 ; his distances up to 8" seemed all 

 to be desired. Dembowski's measurements of angles also re- 

 quired no correction, but his distances, especially about 6", 

 demand a small positive correction (o"22). In the second volume 

 one finds the measures of W. Herschel's classes V. and VI., 

 couples with large proper motion, including measures for the 

 determination of parallaxes, and for the determination of the 

 relation of the number of optical to physical binary, stars dis- 

 covered by M. Burnham and other astronomers, and a continu- 

 ation of W. Struve's and O. Struve's measures. Double-star 

 astronomy is already possessed of two fine monuments in the 

 works of W. Struve's " Mensurse Micrometricas " and of Baron 

 Dembowski's " Misure Micrometriche," and to-day we may, 

 as M. Bigourdan adds, name a third in the " Mesures Micro- 

 metriques des Etoiles Doubles" of M. Otto Struve. 



Method of Pivot Testing. — By means of interference 

 fringes, employed by M. Fizeau in his researches on crystals, 

 M. Maurice Hamy describes a method of studying the form of 

 pivots of a meridian instrument {Comptes Rendiis, No. 20, 

 Nov. 13th), which indicates errors not discernible by the ordinary 

 course adopted. The great advantage to be gained by it is 

 that the state of the pivots can be very easily, and with the 

 expenditure of a very little time, ascertained. The arrange- 

 ment consists in placing a metallic block astride a pivot, the 

 block being supported further by a pointer fixed to a part of 

 the telescope. The extremity of this pointer fits into the 

 bottom of a horizontal groove, parallel to the meridian, in the 

 pier. Contacts between the pivot and the pointer is thus en- 

 sured by the pressure of several weights, while displacements 

 iOf the whole arrangement against slipping are totally eliminated. 

 On the block rests, at one of its extremities above the centre of 

 the pivot, a lever which is movable about an axis on the pillar 

 on a vertical plane ; this carries a small horizontal piece of 

 glass, fixed in a certain manner. Between this mirror and the 

 front of the lens of a fixed collimator are produced the inter- 

 ference fringes, the source of light (monochromatic) being 

 placed at the focus of the lens. Turning the telescope on its 



NO. 1257, VOL. 49] 



axis, the block remains still, but movements of a small nature 

 in the vertical direction were observed which were sufficient to 

 indicate the imperfectness of the pivot. To obtain at a glance 

 the order of the magnitude of such errors, a plane mirror was 

 fixed at some distance from the axis of the lever, so determined 

 that the fringes were displaced by a row when the inclination 

 of the telescope experienced a perturbation of O'Ois. by the 

 action of one of the irregularities. The method of observation 

 consists simply in counting the number of fringes which exceed 

 a fixed limit when the telescope is turned, the number thus 

 obtained expressing in hundredths of a second in time the order 

 of the error. A trial of the above method shows that irregu- 

 larities on the surface of pivots can be easily observed, and, 

 moreover, the errors " ne sont pas completement negligeable 

 au point de vue des observations." 



A Bright Meteor. — The following are a few facts about a 

 bright meteor which Prof. Schur, of Gottingen, has been good 

 enough to send us : — The meteor was observed on Monday, 

 November 27, at 5h. 54m. mean time, and the direction of its 

 path lay between ^ Perseii and towards a Piscium. At first it 

 appeared as bright as a Tauri, and then quickly excelled Jupiter 

 in brilliancy, the light gradually fading away afterwards. The 

 duration of the phenomenon was estimated at about ten seconds, 

 and the trail was observed to be of a yellowish-red colour. 

 Curiously enough, three minutes later a fainter meteor shot 

 across the heavens from the zenith, its direction being nearly 

 at right angles to that of the preceding one. 



Astronomical Photography. — Mr. H. C. Russell, 

 F.R.S., President of the Astronomy, Mathematics, and Physics 

 Section of the Australasian Association for the Advancement of 

 Science, traced the history of astronomical photography in his 

 presidential address at the recent Adelaide meeting. " In many 

 departments of astronomy," he declared in the opening para- 

 graph, " the observer must stand aside while photography takes 

 his place and works with a power of which he is not capable, 

 and I feel sure that in a very few years the observer will be 

 displaced altogether, while his duty will be done by a new sen- 

 sitive being — a being not subject to fatigue, to east winds, to 

 temper, and to bias, but one above all these weaknesses, calm 

 and unrufHed ; with all the world shut out, and living only to 

 catch the fleeting rays of light, and tell their story." 



" Vierteljahrschrift der Astronomischex Gesell- 

 schaft." — The third part of this year's publication gives an 

 account of the work done at the observatories usually included 

 in this list, each director, as has been done in former numbers, 

 summing up in a few words, and stating the work being, and 

 about to be, accomplished. We must refer our readers to the 

 publication itself for individual information. 



GEOGRAPHICAL NOTES. 



UAfrique gives a brief account of the last exploring journey 

 of the late M. Georges Muller in Madagascar. He had returned 

 to Antananarivo from a successful visit to Antsirabe, where he 

 went to collect bones of epiomis, and in June he set out for Lake 

 Alaotra, which, in company with Father Roblet, he explored, 

 adding a number of features to the maps of the district. Parting 

 from his companion, Muller pushed on with the view of reaching 

 Mojanga on the west coast, but near Mandritsara he was 

 attacked and murdered by a party of Fahavalos, one of the 

 independent tribes who still contend against the Hova supre- 

 macy of the island. 



The Madras Mail says that the Indian Marine Survey vessel 

 Investigator has proceeded to the Laccadives to continue the 

 survey of those islands, which has been in course of preparation 

 during the last two years. From the Laccadives the Investigator 

 will go to Madras, and will be engaged for a few weeks in 

 completing the East Coast Marine Survey from Pulicat Lake, 

 where work was left off' last year, to Madras Harbour. Finally 

 in February the Investigator will proceed to Palk Straits, and a 

 thorough survey of the dividing sea between India and Ceylon 

 will be made, ostensibly with the object of testing the practic- 

 ability of constructing a canal and a railway. The distance 

 from the Indian mainland to Ceylon is sixty miles, of which 

 twenty constitute Adam's Bridge proper. The bridge is said to 

 consist of an irregular ridge formed of rock and sand partly dry 

 at low water, but intersected by small intricate channels navi- 

 gable only for native boats of very light draught. Average 



