i6o 



NATURE 



[December 13, 1894 



These additions have enlarged the book from 132 to 181 

 pages, and have correspondingly increased its value. It would 

 be difficult to find an elementary class-book of commercial 

 geography constructed on better lines, or in which the informa- 

 tion is more concisely and accurately stated. 



The additions to the Zoological Society's Gardens during the 

 past week include a Spotted Owl {Athene brama), four Grey 

 Francolins {Francolinus /•onticerianu!), three Rain Quails 

 Coturiiix eoromanJelica), an Indian House Sparrow (Passer 

 dameslicus), two Redheaded Buntings {Emteriza luleola), two 

 Nutmeg Finches (Miitiia riibro-iiigra), a Spotted Turtle Dove 

 (Turtur meenti) Uom India, presented by Mr. E. W. Harper, 

 a West African Love Bird [Agafornis pullaHa) from West 

 Africa, presented by Mrs. Robinson ; a Reticulated Python 

 (Python reticulatus) from Malacca, presented by Mr. Sigis- 

 mund Bruzaud ; an American Black Bear ( Ursus americanus, 

 var. cinnamonia) from the Rocky Mountains, two Common 

 Cassowaries (Casuarius galeatiis) from India, a Red-vented 

 Parrot (Pionus menstniiis), two Orange-flanked Parrakeets 

 (Brotogerys fyrrhopterus) from South America, a Scops Owl 



(Scops ) from Formosa, deposited ; a Short-billed Toucan 



{Ramphaitos brevicarinatus) from Central America, received in 

 exchange. 



OUR ASTRONOMICAL COLUMN. 



Motion .^nd Magnitude — Students of elementary astro- 

 nomy nften believe that stellar motions in the line of sight 

 soon produce changes in the magnitudes of stars. Motion 

 towards the earth involves, of course, a certain increase 

 of magnitude, and a motion of recession must carry 

 with it a decrease, but the amount in the case of a star 

 is far too small to be measurable, even if the magnitudes 

 are observed during many generations. At the meeting of 

 the Amsterdam .\cademy on November 24, Prof Oudcmans 

 communicated the results of an investigation to determine exactly 

 how long stars of which the velocity in the line of sight are 

 known would have to go on moving, in order to produce a change 

 of 01 magnitude. Fiom his own list of parallaxes in vol. 122 

 of the Aslr. Nachriihten, and Vogel and Scheiner's list of proper 

 motions in the line of sight (" Potsdam Observations," vii. i. 

 p. 153, 154), fourteen stars were selected, four of them receding 

 from, and the remaining ten approaching, the solar system. 

 Adopting a solar parallax = 8"'8l 5, and the logarithm of the pro- 

 portion of the increase of brilliancy for one magnilude = 0400, 

 he found that the period required is given by the formula 



years for stars that are receding from, 

 parallax x motion 



and 



5916 



years for those that are approaching 



parallax motion 

 to the solar system ; the parallax being expressed in seconds, 

 and the motion in geographical miles (l geographical mile 

 = 4'6i English miles). Aldebaran proved to be the only star 

 of which the brightness could, since Ptolemy's time, have ex- 

 perienced a loss of 01 magnitude by its radial mo'.ion, provided 

 that the parallax o '52, found by Otto Struve, is trustworthy. 

 Elkin found a value - o''i2, and adopting this value, the period 

 becomes 4\ times longer. For the other stars the result was 

 5500 years at least for Procyon ; while most of them gave ten- 

 'bousands of years as the result. 



The Recent Transit of Mercury.— Details of several of 

 'he American observations are puSlished in Astronomical 

 Journal, No. 330. At most of the stations, some of the 

 <:ontact9 could not be observed on account of clouds, but, on 

 the whole, a fair amount of success attended the observers. 

 Prof Voung observed ihe fir-t and second contacts, and reports 

 that there was a sort of "hardening " of the sun's limb close to 

 the expected point of first contact a few seconds before the actual 

 contact look place ; he has now observed this phenomenon 

 four times, and states that " it may be due to the planet's 

 obscurvaiion of the brighie<l p.irt of the chromosphere close to 

 the disc of the sun, or to some difTrjction effect at the limb of 



the planet." A large " black drop " was observed two seconds 

 before the second contact. Prof. Todd failed to observe the 

 contacts on account of unfavourable weather, but obtained some 

 results during the passage across the disc. He says :— 

 " The planet appeared perfectly circular, sharply defined about 

 the limb, unattended by any halo or atmospheric ring, and of 

 the same colour as the unibrx of the spots on the sun. No 

 attendant satellite of Mercury was seen, nor could any bright 

 spot be discerned upon the centre of the disc, although intently 

 looked for. Any satellite smaller than 100 miles would 

 have escaped detection." 



The New Achromatic Object-gl.ass. — Engineering for 

 November 23 and 30 contains a full account of a fine I2.\ inch 

 equatorial that has just been built by Messrs. Thomas Cooke 

 and Sons, York, for the new observatory at Rio de Janeiro, 

 together with a sketch of the Buckingham Works at 

 York. The description is accompanied with twenty-nine 

 detailed drawings of the instrument. It appears that the 

 works are the only ones in this country where every part 

 of a modern astronomical telescope is constructed ; with the 

 exception of the actual making of the glass, everything is 

 done on the premises, even the heavy 

 cast-iron pillars being made in the 

 firm's own foundry. The new instru- 

 ment, with Messrs. Cooke's standard 

 form of equatorial mounting for large 

 refracting telescopes, is fully equipped 

 for all branches of astronomic.il 

 research. The clock, too, has all 

 the latest improvements for adjust- 

 ment. The object-glass is of the three 

 lens form, invented by Mr. Taylor, 

 who deserves the thanks of astro- 

 nomers for producing an objeciive 

 which, whilst made of durable kinds 

 of glass, and of moderate focal length, 

 is perfectly achromatic. How this 

 end is attained was described in these 

 columns on March 15 {\o\. xlix. p. 

 464). A section of the lens is shown 

 in Fig. I. The first, Lj, is made of 

 baryta light flint, having a refractive 

 index of I "5637 for the I) ray, and 

 its reciprocal of dispersive jiower is 

 50'6. The second lens, l ^., is made 

 of anew glass known as boro silicate 

 flint. This glass has a refractive 

 index of l •54685 for the D ray, and 

 the reciprocal of its dispersive power 

 is 50*2. The third lens, 1.3, is made 

 of light silicate crown glass having 

 the following characteristics : re- 

 fractive index for D ray, I '5109 ; re- 

 ciprocal of dispersive power 60*4. By 

 suitably proportioning the various 

 radii of the lenses, the middle nega- 

 tive lens may be made to almost 

 ex.ictly compensate for the dispersion 

 effected by the other two, and this 

 without necessitating any exception- 

 ally great focal length. If necessary this may be made so 

 small as fifteen times the aperlurc, and there is no difficulty 

 whatever in making a glass with a focal length of eighteen 

 apertures. A small space is provided between the second 

 and third lens, the object of this being to make the 

 correction for spherical aberration as complete as possible ; 

 and when the thickness of this space is properly proportioned, 

 there is an entire al>sence of spherical aberration for all colours 

 of the si)ectrum. Object-glasses made on this plan have been 

 tested by astronomers for visual and photographic observations. 

 In both cases the stellar images showed no indications of 

 residual colour, the lenses behaving just like the mirror of a 

 reflector. 



Lphemeris for Swift's CuMet. — The surmise noted in 

 these columns last week has proved to be correct. M. Schulhof 

 shows pretty conclusively, in the current Coinptei-renJus, that 

 Swift's new comet is really identical with I)e Vico's comet 

 1844 I. The subjoined epliemcris is from one given by Prof. 

 Lamp in the Astronomischc Naclitichtcn, No. 326O. 



Fig. I.— Section of the New 

 Object-gl.iss. 



NO. 13 II, VOL 51] 



