2SO 



NA TURE 



[January 22, 1903 



OUR ASTRONOMICAL COLUMN. 



New COMET 1903a (GlACOBINl). — A telegram from the 

 Kiel Centralstelle states that M. Giacobini, observing at Nice, 

 has discovered another new comet, the position of which at 

 6h. 2Sm. '9 (Nice M.T. ) on January 19 was as follows : — 



K.A. = 22h. 57m. 48s. 

 Dec. = + 2° 16' 24" 



that i=, between the stars and 2 Piscium, about one-third the 

 distance from #. 



The daily movements in R. A. and Dec. are + 17' (arc) and 

 - 12' respectively. 



Comet 1902 d. — No. 3838 of the Astronomische Nachrichten 

 contains several sets of observations of this comet, and the 

 elements and ephemeris given below. The latter have been 

 calculated by Herr F. Ristenpart from the means of three 

 observations made on December 3, three observations made on 

 December 1 1 and of two observations made on December 23, 

 all of them having been made by different observers : — 



T = 1903 March 25-32785 M.T. Berlin. 



The Relation between Faculae and Prominences. — 

 InNo. 11, vol. xxxi., of the Memorit delta Societa degti Spettro- 

 scopisti Italiani, Signor Mascari submits a large number of 

 arguments and data in order to show that there is no real con- 

 nection between the solar hydrogen prominences and faculae, 

 but that where facul.e are attended by other outbursts, these 

 outbursts are of the eruptive prominence type. 



For instance, in 1895, the prominences had a maximum 

 frequency at +30° to +40 and -20° to - 30 (heliographic 

 latitudes), and strong minima at +6o : to +90^ respectively, 

 whilst the groups of facula; showed an almost symmetrical 

 arrangement with regard to the solar equator, having only slight 

 maxima of frequency at + IO° to +20' and + 70° to +80° ; the 

 same relations held during 1S96, and many similar cases are 

 quoted by the writer for other years. 



Again, out of two hundred and ninety-six groups of faculae 

 observed in 1900, only fourteen were coincident with ordinary 

 prominences, whilst ninety-nine coincided with eruptive 

 prominences having bases of small extent. 



Signor Mascari therefore arrives at the conclusion that the 

 hydrogen prominences, such as are commonly observed on the 

 sun's limb, and facuke are two distinct and completely inde- 

 pendent phenomena. 



Spectrographic Determination of the Rotation 

 Period of Jupiter. — Two excellent spectrograms of Jupiter, 

 obtained by Mr. V. M. Slipher, of the Lowell Observatory, 

 Flagstaff, Mexico, are reproduced in No. 101 of Popular 

 Astronomy. No. 1 was taken in such a position that the dis- 

 persion was parallel to the equatorial diameter of the planet, 

 whilst in No. 2 the dispersion was parallel to the polar 

 diameter. 



No. 1 shows a very distinct displacement or inclination of the 

 lines in the planet's spectrum as compared with the lines in the 

 lunar spectrum, whrch was photographed as a comparison 

 spectrum on boih sides of it. In No. 2, this displacement was 

 nonexistent. Measurements of the displacement in spectrogram 

 No. 1 were made, and, on applying Doppler's principle to them, 

 values for the rotation period which are well in accordance with 

 the accepted values were obtatned. 



The Photography of Stellar Regions.— In a paper 

 recently communicated to the Vienna Academy of Sciences, Herr 

 Egon von Oppolzer discusses the question as to how the greatest 



NO. 1734, VOL. 67] 



number of star images may be obtained when photographing 

 stellar regions. 



He points out that in an objective uncorrected for curvature 

 of the focal surface, this surface is a sphere having its centre in 

 the centre of the objective, and it will only be on the intersection 

 curve (a circle) of this sphere and the photographic plate that 

 the star images will be in focus, and therefore it will only be on 

 this curve that images of the fainter stars will be obtained. 

 The further from the curve the star image happens to fall, the 

 greater will have to be the magnitude of the star in order that 

 its image may be photographed. 



Herr Oppolzer then establishes a relation connecting the 

 distance of a star image from this focal circle, the radius of the 

 focal circle and the focal length of the objective, and finally 

 arrives at the conclusion that the formula 5 = L2/l6/(where L = 

 the length of the side of the square plate and /"= the focal 

 hngth of the objective) gives the best distance (5 ) that the plate 

 must be pushed in from the axial focus in order that the maxi- 

 mum number of stars may be photographed. Applying this 

 formula to the Potsdam astrographic refractor, we find that the 

 plate should have been pushed in 0-47 mm., whereas we see, 

 from the Potsdam plates for the "Carte Celeste," that it was 

 only pushed in 013 mm., and Herr Oppolzer deduces from 

 this that an unnecessary loss, amounting to as much as 6 per 

 cent., has occurred in the number of stars photographed 

 (Astrophysical Journal, No. 5, vol. xvi.). 



THE FORMATION OF PEARLS} 



DV far the greater number of recent writers on pearls, whether 

 scientific or otherwise, when discussing the cause of pearl- 

 formation, have contented themselves with recapitulating what 

 has already been written on the subject, without attempting to 

 verify or refute the various hypotheses that have been pro- 

 pounded. The question is one which has called forth specu- I 

 lalive theories since the earliest times of which we have any 

 records ; but, with the exception of the brilliant researches of 

 Filippi and a few of his contemporaries, theory has prevailed to 

 the almost complete exclusion of practical investigation. 



In a recent paper, 1 based upon an examination of large 

 quantities of material comprising a number of the pearl-pro- 

 ducing species of mollusca, I have tried to place our knowledge 

 of the matter upon a more satisfactory basis. 



By observations upon pearl-bearing examples of the common 

 mussel, Mytilus edit/is (which were confirmed in the case of all 

 other species examined), I proved that the formation of the 

 pearl takes place in exactly the same way as that of the shell, 

 except that a true pearl is laid down in a closed sac of the shell- 

 secreting epithelium, .embedded in the subepidermal tissue of 

 the mantle and completely cut off from the outer epithelium 

 itself. Inside this spherical epithelial sac, the shell substance is 

 laid down in the concentric layers that are so characteristic of 

 the pearls, instead of in the parallel lamellae which are found in 

 the shell itself. Such a sac, with its contained pearl, may be 

 compared to a human atheroma cyst. 



1 his makes it necessary for us to draw a sharp distinction 

 between pearls proper and blisters or pearly excrescences of the 

 shell lining, which are secreted by the outer (shell-lorming) 

 mantle epithelium, to cover over foreign bodies that have in- 

 truded themselves between the mantle and the shell or to 

 repair the damages done by shell-boring domiciliaries. " Con- 

 cretions" are, again, distinguished from pearls as calcosphxritic 

 bodies which have not a cuticular origin from an epithelium, 

 but seem to arise by free crystallisation in the mantle or other 

 tissues. The term "attached pearl " should be applied only to . 

 pearls which have become secondarily fused Jo the shell by 

 absorption of the intervening tissues. 



From the facts of pearl-formation, it is easy to understand 

 why the pearl presents the special characters of the par- 

 ticular species of shell from which it is taken, and also why, in 

 the same mollusc, the characters of the pearls produced are 

 determined by the part of the mantle in which they are formed. 

 Thus, pearls formed in the extreme mantle margin are composed 

 mainly of periostracum, e.g. the leathery pearls of Mcdiola 

 modiolus, while those which occur in the part of the mantle 

 concerned in depositing the prismatic substance are made up of 



1 " On the Origin of Pearls."' By Dr. H. Lyster Jameson (Proceedings of 

 die Zoological Society of London, 1902, vol. i., pp. 140-1^6, pi. xiv-xvir.). 



