42 



NA TURE 



\_Nov. 12, 1885 



OUR ASTRONOMICAL COLUMN 

 The Rotation Period of Mars. — The seventh volume of 

 the Annals of the Leiden Observatory contains a very thorough 

 and painstalcing investigation by Prof. Bakhuyzen of tlie 

 rotation period of the planet Mars. In previous determinations 

 one of two courses has usually been adopted, either to compare 

 drawings of Huygens or Hooke with the most recent observa- 

 tions attainable, or to discuss some modern series which seemed 

 to promise to co.iipensate for its restricted lange by its greater 

 accuracy. Prof. Bakhuyzen has, however, endeavoured to 

 utilise the entiie mass of observations at his disposal, so as to 

 avoid the sources of error to which the other methods are liable, 

 and he possesses a great advantage over earlier investigators, in 

 having access not only to the numerous observations made in 

 1877 and 1879, but al;o to the great series of more than 200 

 drawings which Schroeter had prepared for his projected " Areo- 

 graphischen Beitrage," and which, becoming the property of the 

 University of Leiden in 1S76, was edited and published by Prof. 

 Bakhuyzen in iSSi. Prof. Bakhuyzen, in the reduction of these 

 drawings, has adopted provisionally Schiaparelli's position for 

 the pole of Mars — R. A. 317° ^d'-o, Dec. 53° 25' '4, mean equinox 

 of i833"0 — and Proctor's rotation- period— 24h. 37m. 2274s. — 

 and deduces corrections to these elements from a comparison of 

 the results obtained by reducing the various observations at his 

 command with them. His first step is, from a discussion of the 

 drawings of Kaiser, Lockyer, Lord Rosse, and Dawes, made 

 during the oppositions of 1862 and 1S64, to obtain the time of 

 transit on January i, 1863, of his adopted prime meridian over 

 the Martial meridian which passes through the earth's north 

 pole, choosing as his prime meridian the one which lies 2° to the 

 east of the centre of Madler's point a, corresponding almost 

 exactly to 'Achl^fpsxeWW Fasii^inm Aryn, or to Proctor's Daivis 

 Forked Bay, he finds the time of transit over the meridian passing 

 through the north pole of the earth on January i, 1863, to be 

 2oh. 27 om. ± 40m, Berlin M.T. The areographic longitude 

 of the centre of the Oailns, the conspicuous circular .spot, called 

 by Green the Terby Sea, and by Schiaparelli Lacus Soils, 

 will be, with this prime meridian, 90'S7°. The second 

 section contains the determination of the areographic 

 longitudes of ten of the most conspicuous and easily 

 identified markings on the surface of Mars as inferred 

 by means of the above elements from the drawings of various 

 observers from the time of Hooke and Huygens up to 1S79. 

 For the last-named year only Schiaparelli's observations are 

 used, but for 1877 there is an abundant supply, there being 

 available, besides the observations of Schiaparelli, the drawings 

 of Lohse, Green, Dreyer, and Niesten. Beer and Madler's 

 drawings aflbrd material for 1830, Herschel and Schroeter give 

 a very full series from 1777 to 1803 ; and Huygens and Hooke 

 supply a few drawings from 1659 to 16S3, from which the longi- 

 tude of Madler's/, the Kaiser ox Hour-^lass Sea, Schiaparelli's 

 Syrtis Major, can be inferred. These longitudes are discussed 

 in the third section, and a corrected rotation period is obtained 

 of 24h. 37ni. 22'66s ± o'oi32s., a value exceedingly close to 

 the mean of the best previous determinations, which are as 

 follows : — 



Kaiser, 1S64 

 Kaiser, 1873 

 Schmidt, 1873 

 Proctor, 1868 



24 37 22-1 



■59> 



•57 



735 



Proctor's value is clearly too large, a comparison of the mean 

 longitudes obtained for the Kaiser Sea with his period showing 

 a steady decrease for successive oppositions ; the only observa- 

 tions which stand conspicuously out from the rest being those of 

 Hooke, upon which he had based his determination. There can 

 be no doubt that I'rof. Bakhuyzen's value is a distinct improve- 

 ment ujjon the earlier ones, and that its uncertainty lies only in 

 the second place of the decimals. A table for computing the 

 time of transit of the prime meridian over that meridian of Mars 

 which passes through the earth's north pole, completes the 

 memoir. 



Prof. Bakhuyzen supplies also a short note as to changes on 

 the surface of Mars. The most conspicuous of all the markings 

 on the planet's surface has always been the Kaiser Sea ; but the 

 drawings of Schroeter and Herschel, as Dr. Terby has already 

 pointed out, exhibit a second marking near it, nearly as con- 

 spicuous, and very similar in shape and size. There can be no 

 doubt that the only modern representative of this spot is Hiti;- 

 :^ns Inlet. Schiaparelli's Cyelopiwi, a narrow streak, by no 



means easily observed, and now entirely unlike the Kaiser Sea in 

 shape. Prof. Bakhuyzen also considers that there is sufficient 

 evidence for thinking that Schroeter on several occasions ob- 

 sen'ed Schiaparelli's Laslrygoniini — one of the most difficult 

 objects on the planet — which could scarcely have been the case 

 had it not been much more conspicuous than it has been of late 

 years. These changes, Prof. Bakhuyzen thinks, lend a high 

 degree of probability to the theory that certain districts of Mars 

 are covered by liquid. 



The Spectrum of the Great Nebula in Andro.meda. — 

 iMr. O.T. Sherman, assistant at Yale College Observatory, reports 

 in Seience (vol. vi. Nos. 1 38 and 141) the discovery of three bright 

 lines in the spectrum of this nebula. Of these the most refrangible 

 corresponds to H/3, and the w-ave-lengths of the other two are 

 given as S3i2'5 and 5594-0. It is suggested that the second of 

 these lines is the well-known coronal line 1474 K, and that the 

 third is one of the feebler coronal lines which Prof. Young 

 observed in the 1869 eclipse, viz. the one at 1250 ± of Kirch- 

 ho2"'s scale. The observation, if confirmed, will go far to settle 

 the disputed question as to whether the Nova is really or only 

 apparently connected with the nebula, for two bright lines, of 

 which one is probably 1474 K, have been observed in the spec- 

 trum of the former at the Royal Observatory, Greenwich. 



The Wedge Photometer. — Dr. Wilsing in the Aslroiio- 

 ?iiisehe Naehriclilen, No. 2680, criticises at considerable length 

 several points with regard to Prof. Pritchard's use of the wedge 

 photometer at the Oxford Observatory. Dr. Wilsing considers 

 that Prof. Pritchard's investigations as to the figure of the wedge 

 and its selective absorption leave nothing to be desired, but that 

 the state of our knowledge of the physiological side of the ques- 

 tion is ' still very incomplete. Experiments w hich Dr. Wilsing 

 has made with two wedges of his own have convinced him that 

 the variations in the sensibility of the eye are neither slight nor 

 unimportant, and that they occasion discordances in the obser- 

 vations considerably greater than Prof. Pritchard is inclined to 

 admit. Dr. Wilsing also finds that comparisons of differently 

 coloured stars give results not directly comparable with eye 

 estimations. He objects to theu-eof the method of limiting 

 apertures for the determination of the value of the wedge con- 

 stant, and points out that the influence of the intensity of the 

 background affects Prof. Pritchard's magnitudes of the fainter 

 stars very percepiibly. Despite all these drawbacks, however, 

 he regards the wedge photometer as a useful addition to our 

 equipment. 



Mr. Chandler, who must at Harvard College enjoy special 

 facilities for making himself well acquainted with the working ot 

 different forms of photometers, has recently expressed his pre- 

 ference for Argelander's methoti. There can, however, be no 

 doubt but that the labours of Profs. Pritchard and Pickering 

 have greatly advanced our knowledge of the comparative bright- 

 nesses of the northern stars. 



Photographing the Corona in Full Sunshine.— Mr. 

 W. H. Pickering, of Harvard College, made a series of 

 attempts duringthe partial eclipse of last March 16, to oblain a 

 photograph of the corona. In this he was quite unsuccessfid, 

 for, though his plates showed several corona-like markings, they 

 were clearly not due to the true corona, as they were found in 

 front of the moon as well as on the sun's limb. From tliis Mr. 

 Pickering was evidently led to conclude that the results which 

 Dr. Huggins had obtained were probably of a similar character, 

 and he expressed as much in a letter to Science. Dr. Huggins 

 in reply pointed out that Mr. Pickering's method was faulty 

 and was calculated to produce such false images. The latter, 

 therefore, somewhat modified his apparatus, without, however, 

 altering the two points which Dr. Huggins considered most 

 erroneous — viz. the use of an object-glass instead of a reflector, 

 and the placing his drop-slit close in front of the object-glass 

 instead of in its primary focus. The result has been that he has 

 obtained photographs free from false coronx, but showing no 

 real ones. At the same time he has made experiments which 

 convince him that to produce a perceptible image of a coronal 

 rift it is necessary to be able to discriminate between degrees of 

 illumination which do not differ from each other by more than 

 one-tenth the intrinsic brilliancy of the full moon. He con- 

 siders that the eye is more able to detect small differences of 

 light than a photograph is, and states that the moon cannot be 

 photographed in full daylight, even though it may be easily 

 seen. His investigations also lead him to think that even in the 

 clearest weather the atmospheric illumination is 300 times as 



