February 27, 1896] 



NATURE 



403 



The additions to the 2k>ological Society's Gardens during the 

 past week include a Lion {Felis ho, 6 ) from Africa, presented 

 by Mr. Rowland Ward ; a Common Squirrel (Sciurus vul- 

 garis), British, presented by Mrs. Herbert Morris ; a 

 Woodlark (Alauda arhored) ; a Whinchat {Pratincola rnbetra), 

 British, presented by Mr. J. Young ; a Black Tanager 

 {Tachyphonus inelaleuciis) from South America, presented by 

 Madame Gate ; a Rhinoceros Hornbill [Buceros rhinoceros, <J ) 

 from the Malay Peninsula, deposited ; two Black- necked Stilt- 

 Plovers (Hintautopus nigricollis) from South America, a Long- 

 eared Owl {Asio otus), two Common Pheasants (Pkasianus 

 iolchicus, 9 9), British, purchased. 



OUR ASTRONOMICAL COLUMN. 



Cassegrain and Gregorian Reflectors. — For some time 

 past. Prof. Schaeberle, of the Lick Observatory, has been experi- 

 menting on the applications of these rather neglected forms of 

 reflecting telescopes with the special view of applying them for 

 celestial photography. The field of view in such instruments is 

 darker than in refracting telescopes, and if the mirrors are good, 

 there is every reason to expect that in planetary photography the 

 results obtained by a telescope twelve or fifteen feet in length 

 will be at least equal to those obtained with the most powerful 

 refractors. There is so little published on the theory of the 

 instruments that Prof Schaeberle has re-investigated the 

 fundamental formulae, which may be useful to many observers 

 {Astronomical Journal No. 364). In the case of the Casse- 

 grain it is easily shown that the secondarj' mirror must 

 theoretically be the convex side of an hyperboloid of revolution, 

 while in the Gregorian it must be the concave side of an 

 ellipsoid of revolution ; in both cases the axis and focus of 

 the secondary reflector must coincide with those of the primary. 

 If F denote the focal length of the parabolic reflector, H the 

 distance between the centre of the mirror and the secondary 

 focus (being negative when it falls between the two reflectors) ; 

 the axial radius of curvature of the secondary reflector ; L the 

 equivalent focal length of the combination ; then, for both 

 f)rms 



L = - -^ (F -t- H) ± «^r^-f-TF~+ll)2} 



_ 2LF(F^-f- H) 



(I) 



(2) 



H 



(3) 



^(L^ - F^) _ p 



2LF 



These formulae refer to the theoretical conditions for perfect 

 ilcfinition ; in practice, the secondary mirror is so small that for 

 the same value of r, a spherical surface, or a paraboloid, hyper- 

 boloid, or ellipsoid, of any eccentricity, gives tolerable images, 

 the size of which may be varied by simply moving the secondary 

 along the optic axis. There will, however, always be a point of 

 best definition ; and if this does not give a convenient position 

 for the secondary focus, the eccentricity of the small reflector 

 must be altered by local polishing. The figuring of the secondary 

 mirror is too delicate for direct measurements, and can only be 

 tested in the telescope itself. 



Wells' Algol Variable. — Further particulars of the new- 

 variable of the Algol type in the constellation Delphinus 

 (Nature, vol. liii. p. 206) are given in Harvard Observatory 

 Circular, No. 5. The observations so far obtained show that 

 its time of minimum, uncorrected for the velocity of light, can 

 be closely represented by the formula J. D. 24 1 2002 '5 -I- 4 "8064 E. 

 For nearly two hours before and after the minimum it is fainter 

 than the twelfth magnitude, but it is not yet possible to say how 

 faint it really becomes. The increase of brightness takes place 

 at first very rapidly, but afterwards more slowly, and the full 

 brightness, magnitude 95, is reached in about five hours. 

 Numerous photographs indicate that during the four days between 

 the minima, the brightne.ss is sensibly constant. The changes 

 can be explained on the assumption that the star revolves about 

 a relatively dark body, and that it is totally eclipsed at the 

 minimum, the light then, if any, being that of the companion. 

 The conditions are accordingly somewhat similar to those of 

 U Cephei. The new variable is BD-H7°4367, its R.A. and 

 decl. for 1900 being respectively 20" 33''l and -f 17° 56'. 



NO. 1374. VOL. 53] 



The New Comet. — From observations of the new comet, 

 Perrine-Lamp, made on February 15, 16, and 17, the following 

 elements and ephemeris for Berlin midnight have been computed 

 by Dr. Lamp : 



T= 1896 January 31 "999 Berlin mean time. 



<^=is^ 38 16] 



4i=2o8 36 29 1896-0 

 ?=I55 30 15) 

 log q =977022 



R.A. Decl. Brightness, 



h. m. s. o , 



Feb. 27 .. 22 59 40 ... -f 46 87 

 ,, 29 ... 23 54 31 ... -I- 49 492 ... 0-9 

 M. Bigourdan, who observed the comet at Paris on February 

 16, states that the comet was not then visible to the naked eye, 

 though bright enough to be easily seen in a telescope of 50 mm. 

 aperture. It was observed to be round, about 2 in diameter, 

 with a diffuse central condensation about 20" in extent. At the 

 centre of this condensation a small stellar nucleus was occa- 

 sionally suspected. No tail was visible. 



Perrine's Comet (1895).— Dr. E. Lamp has drawn our 

 attention to a misprint in the ephemeris of Perrine's comet, given 

 in Nature on January 23 ; the declinations throughout should 

 be placed a line lower. Hence our statement last week, that on 

 re-discovery the declination of the comet was a degree in 

 error was unfounded ; as a matter of fact, the corrections to the 

 ephemeris were only -f 4s. and -J- ©'-5. The following is a 

 corrected and extended ephemeris : — 



R.A. Decl. Bright- 



h. m. s. o - ness. 



Feb. 29 ... 19 46 48 ... + I 55-1 ... 0-12 



Mar. 4 ... 46 41 ... 3 0-7 ... o-ii 



,, 8 ... 19 46 16 ... -f 4 6-7 ... o-io 



M. Bigourdan describes the comet as being round, with a 

 diameter of about 50", and showing a somewhat diffuse nucleus. 

 It is seen with about the same facility as a star of the twelfth 

 magnitude. 



MAGNETIC SURVEYS?- 



ON looking through these two volumes, the first thought that 

 strikes one is that magnetical observations require a great 

 deal of time for reduction and preparation for publication. In 

 the Russian work, this great consumption of time is perhaps 

 to some extent explained by the distance that separates the place 

 of observation from any centre of civilisation, whence the results 

 could be published, and the inevitable difficulties of communica- 

 tion. Perhaps still more is it to be explained by the fact, that 

 several authors have to compile their separate portions under 

 editorial supervision. Still, eleven years does seem a long 

 interval to elapse between the completion of the observations and 

 the distribution of the results to the public. In the Italian work ; 

 carried out in the cultured and accessible cities of Italy, six 

 years have sufficed for the reduction and the printing. 



This remark must not be construed as expressing any wish to 

 minimise the difficulties that a scientific expedition to the wilds of 

 North-East Siberia must of necessity encounter, or to make light 

 of the dangers that these enterprising officers experienced in 

 their expatriation, cheerfully endured for scientific ends. One 

 disaster that these scientific experts had to undergo may be 

 mentioned, as it illustrates not only the severities under which 

 they were placed, but the readiness of resource with which they 

 remedied the mishaps, far away from trained workmen and 

 mechanical apparatus. On August 6, 1882 (about the time of 

 the Fort Rae Expedition, it may be remarked), when nearing 

 their destination, they experienced a north-east storm of more 

 than an unplea.sant character, which carried their craft on to the 

 rocks, and tumbled their apparatus into the water. The chest 

 containing the instruments for observing the magnetic variation 

 at Ssaga.styr (Long. E. 8h. 26m. ; Lat. 73°23'N.) remained 

 some hours at the bottom, before it could be successfully landed, 



1 " Beobachtungen der Russischen Polarstation an der Lenaraiindunt;. ' 

 I. Theil Astronomi.sche und Magnetische Beobachtungen 1882-1884 " 

 Bearbeitet von V. F'uss, F. MQller und N. Jurgens. Herausgegeben von 

 Dr. A. V. Tillo, 1895. . . 



" Misure Assolute degli Element! del Magnetismo Terrestre, eseguue in 

 Italia negli Jinni 1888 e 1889 dal Dott. Luigi Palazzo." (Roma, 1895.) 



