426 



NATURE 



[Sei'Temukk 3, 1896 



(Stcnorarins crcfidatus) from Spitzbergen, presented by Mr. J. 

 F. Studley ; two \'ariegated Sheldrakes { Taiiorna variegatd) 

 from New Zealand, presented by Sir Walter L. BuUer, 

 K.C.M.G. ; two Streaky-headed Clrosbeaks (Poliospha 

 pilaris) from South Africa, presented by Miss Jessie Porter ; 

 an Oyster-catcher (Hiiiiaiitopm ostralegtis), British, pre- 

 sented by Mr. R. Gurney ; a Bordeaux Snake (Coraiiclla 

 girondtca), a Common Snake {Trofiidonolus natrix, var. ) 

 from France, presented by Mr. E. A. Minchin ; a Squirrel 

 Monkey {Chrysothrix sciiirea) from Guiana, a Beccari's Cas- 

 sowary {Casuariiis heciari) from New Guinea, a Red Kangaroo 

 (Macropus riiftis, 9 ) from Australia, deposited ; two Otters 

 {Liitra vulgaris) {torn Ireland, four Cayenne Lapwings ( Vanelliis 

 caj'eniieiisis) from South America, purchased ; a Chinese Mynah 

 (Acridotheres crislatellus) from China, received in exchange ; an 

 African Wild Ass (Eijinis hmiopus 9 )]iborn in the Gardens. 



OUR ASTRONOMICAL COLUMN. 



Double Star Obskrvations. — In Asl. Nadi., No. 3370, 

 Dr. Doberck, while discussing the elements of i\ Coronas 

 Borealis, takes the opportunity to determine the probable error 

 that accompanies the observation of position angle and distance 

 in the case of the better-known double star observers. Three 

 stars have been selected for the discussion. 7) Corona;, a close 

 double, in which the probable errors are referred to a common 

 distance of o"'7 ; o Cent.iuri, reduced to a mean distance of 10", 

 and, of course, including a different class of observers ; and 

 7 Virginis, with a mean distance of 2"'5. Dr. Doberck might 

 with advantage have given the aperture of the telescope with 

 which the observations have been made, but a glance at the list 

 is sufficient to show that the greatest accuracy, as might have 

 been anticipated, is on the side of the large telescopes. In the 

 case of r) Coronne, Profs. Hall and Burnham are the only 

 observers whose probable errors fall below l^ in position angle. 

 In distance, their only competitor is M. Perrotin, who also has 

 the advantage of large aperture. With 7 \'irginis, where the 

 components are more widely separated, telescopes of moderate 

 size are able to compete advantageously, and the measures of 

 MM. Duner and Schiaparelli appear quite as trustworthy as 

 those of Prof. Hall. The probable errors attached to observa- 

 tions made in the southern hemisphere are, on the whole, 

 slightly larger than those derived from northern observers. 



Variabi.b .Stars. — Owing to the rapid accumulation of new 

 material, which seems to be coming in on all sides. Dr. Chandler 

 thinks that a new edition of his catalogue, incorporating every- 

 thing up to date, is necessary. With this we entirely agree with 

 him ; for, although the system of supplements is a good one, 

 they can accumulate, and when this happens the sooner they can 

 be eliminated the better. Our readers are so familiar with the.se 

 catalogues, that little need be said when it is slated that Dr. 

 Chandler has entirely overhauled the work, and brought in all 

 the new material. It is good, however, to hear, as he says, that 

 the " degree of uniformity and completeness of the observation 

 of the phenomena, and the consequent development of our 

 knowledge with regard thereto, during the past few years is 

 remarkable." He adds, however, further that the need for 

 volunteers in the southern hemisphere is pressing {Astro- 

 nojnical Journal^ No. 379). 



Variable Star Observations. — Profs. Barnard and 

 Chandler have both called attention to possible errors intro- 

 duced into the observation of variable stars from physiological 

 causes. The latter thinks that a systematic error arising from 

 unequal sensitiveness of different portions of the retina, de- 

 pendent upon diflerent positions of the variable star with 

 reference to those with which it is compared at different hour 

 angles, can be traced in the case of the minimum phase of 

 U Pegasi. Mr. A. W. Roberts, in the Astroiioinical Journal, 

 No. 381, urges the employment of some mechanical means for 

 the elimination of this source of error. He himself has been in 

 the habit of using both a negative and a direct-vision eyepiece, 

 and taking the mean of the two observations. In this way, 

 it is asserted, the mean error of observation has fallen from 

 0'I2 mag. to about o'05 mag. The obvious suggestion of 

 employing a prism mounted behind the eyepiece, and taking 

 four observations in such a manner that the comparison star is 

 rotated go' about the variable, is not lost sight of; and when this 



NO. 1 40 I, VOL. 54] 



arrangement is carried into effect, it is confidently anticipated that 

 the mean error will not be greater than 003 mag. It is need- 

 less to point out that this implies a greater degree of accuracy 

 than has been attained with any photometer. The probable 

 error in the case of the Harvard photometer has been quoted as 

 o'075 mag., and Drs. MUUer and Kempf, with the Zfillner 

 photometer at Potsdam, have not been able to make their 

 probable error much below o'o6 mag. Mr. Roberts' experiment 

 will, therefore, be watched with considerable interest. 



The;Cai'E Observatorv. — The Observatory Report for 1895 

 furnishes .several items of general interest. The fine equa- 

 torial, presented by Mr. McClean, is in an adv.anced state, the 

 only part not yet commenced being the line-of-sight spectroscope. 

 Hesides the cylindrical observatory and hemispherical dome, the 

 donor has generously provided a rising floor of excellent design, 

 and also an attached building containing entrance hall, study, 

 developing room, and instrument store. The objective prism of 

 24 inches aperture has been completed by Sir Howard Grubb. 

 The chief part of the year has been occupied in clearing oft 

 arrears of reduction and publication. The publications during 

 the year included — " The Cape General Catalogue for 1885, with 

 Appendices, &c." ; "A Determination of the Solar Parallax and 

 the Mass of the Moon from Heliometer Observations of the 

 Minor Planets Iris, Victoria, Sappho " ; the first volume of the 

 " Cape Photographic Durchmusterung," containing the mean 

 places of 152,000 stars for 1875, derived from Cape photographs 

 between Declinations - 19° and - 37' ; a complete account of 

 the " Geodetic Survey of South Africa." Much actual observa- 

 tional work has been entirely suspended to allow of these publica- 

 tions being completed. It is also mentioned that an increase of 

 staff will be required for the new astrophysical department 

 created by the advent of the McLean lele.scope. With the transit 

 instrument 2872 stars have been observed, the small number 

 being due to the objects being chiefly slow-moving circumpolar 

 .stars. The work with the astro-photographic telescope has been 

 satisfactory. 91 catalogue plates were taken, 55 of these being 

 finally passed. 367 chart plates were exposed, 240 being passed. 

 This leaves 15 catalogue and 253 chart plates yet to be done to 

 complete the complement assigned to the Cape. A complete 

 investigation of the reseau used here (Gautier No. 8), has been 

 made, and will soon be published. The observations made with 

 the zenith telescope in 1892, 1893 and 1894, for aberration and 

 change of latitude, are completely reduced, and are being finally 

 revised. 



An Investigation o.n Aberration and Atmospheric 

 Refractio.n. — The latest volume of the publications of the 

 Washburn Observatory of the University of Wisconsin (vol. ix. ) 

 contains an investigation, by Mr. George C. Comstock, on 

 "Aberration and Atmospheric Refraction." It may be remem- 

 bered that M. L«wy pointed out the extended use of the equa- 

 torial telescope and its adaptation to new lines of research 

 through the introduction of reflecting surfaces in front of the 

 objective. The method adopted here, however, deviates widely 

 from LcBwy's, for reasons given by the author in the introduction. 

 Instead of the employment of a prism in front of the object-glass 

 (the fundamental idea of the apparatus designed by M. Lcewy), 

 the reflecting surfaces of which were the silvered faces of an 

 equiangular glass prism, Mr. Comstock substituted for it three 

 plane mirrors of rectangular cross-section. By this means he was 

 able to overcome the great drawback, met with when using the 

 prism, of the deformations of the prism arising from changes of 

 temperature, and producing errors of focus which seemed to be 

 insuperable with this type of apparatus. .V detailed description 

 of the mirrors and method of mounting, too long to be referred 

 to here, is given. Mr. Comstock next enters on the deter- 

 minations of the errors of the apparatus, and gives tables of the 

 in.strumental constants that follow, a description and investiga- 

 tion of the micrometer employed, and the effect of aberration 

 and refraction upon the apparent distance between two stars 

 respectively. Several other points are investigated, which he 

 found were imporlant after a preliminary trial of the method he 

 adopted. From the discussion of 822 observations of the angular 

 distances separating thirty-nine pairs of stars made by two 

 observers, it appeared, as he says, "that the apparatus as 

 employed is capable of furnishing a very considerable degree 

 of precision, the probable error of a single observation made 

 under normal conditions being + o"'30, i.e. less than a millionth 

 part of the quantity measured." As mentioned before, the ob- 

 servations were made to determine, from the annual variations 



