Jlne, igo6.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



457 



ASTRONOMICAL. 



By Charles P. Butler, A.R.C.Sc. (Lond.), F.R.P.S. 



Determination of Stellar Radial Veloci- 

 ties with the Prismatic Camera. 



Maw aUciniils liavi; biiii inailt- (iLiriii^; llic la>t It-W years 

 to utilib'e thf large light-grasping pDwer of the objective 

 prism for determinations of stellar radial velocities. The 

 chief difficulty is in consequence of there being no known 

 method of introducing a comparison spectrum to give the 

 const.ants from whicfi to measure the displacements. A 

 melliod has recently been described by G. C. Comstock, in 

 which he proposes to employ a specially constructed double 

 prism, which is fi.Kcd with the refracting angles inverted 

 so as to give two overlapping and crossed spectra. By 

 measuring the distance apart of known spectrum lines in 

 the two spectra produced of each of the stars whose 

 velocities are required, and subsequently comparing these 



measures with the distance between simil.ar lines on a 

 standard star, a determination of the velocit\- in the line of 

 sight can be obtained. Asl, ni,h i/sinil -hnuiuil, Mareli, u,vIk 



Results of the American Echpse Expe- 

 dition, August 30th, 1905. 



The expedition was divided into three parties, the mem- 

 bers of which occupied stations at Daroca ftnd Porta Coeli, 

 in Spain, and at Guelma, in Algeria. Prcparatkins were 

 made for photographing the corona with long and short 

 focus cameras, and for spectroscopic photography of the 

 sun's chromosphere and corona. 'Ihirty-six pictures of the 

 corona were obtained at the three stations. Several ex- 

 cellent spectra were photographed with the four-inch para- 

 bolic diffraction grating, ruled with 14438 lines to the inch, 

 and h.'iving a focal length of five feet. The flash spectrum 

 e.xtends from D3 in the yellow, to \ 3300, and shows a 

 great number of lines. With the six-inch grating, the 

 spectrum of the green coronal ring at X 5303 shows most 

 interesting details, at least 15 or 20 small streamers being 

 visible for some distance beyond the moon's limb. A special 

 feature of this corona spectrum is the presence of a ihiil^ 

 streamer, almost radial, and borden-d by bright streamers 

 on either side, which was projected from the moon's limb at 

 latitude 55" or 60", on the sun's east limb. This peculiar 

 feature is also well shown on the 15-foot camera pictures 

 of the corona. 



The Fraunhofer dark crescents, as seen visually with the 

 spectroscope, became visible about seven minutes before the 

 second contact, while Uj and the hydrogen lines became 

 brightly reversed at .about one-and-a-half or two minutes 

 before. It was particularly noticed that the green m.ig- 

 nesium lines persisted until near niid-tot.ality. 



Folariscopic observations were only made at the (iuelm.i 

 station, with very satisfactory results. Inside of 5' of arc 

 there is practically no radially pol.irised light, showing that 

 the luminious matter is, in all probability, solid or liquid, 

 as is further evidenced by the continuous spectrum of this 

 region. Between 5' and 10' of arc, the amount of polarised 

 r.idialion increases very rapidly, indicating th.at this light 

 is ehielly rellected sunlight. — Astrophyairot Jnuriuil, March. 

 1906. 



Parallax of the Nebulae- 



The distances of the nebula- are at present practically 

 unknown, as on the whole they are exceedingly dilVicult to 

 measure exactly, and, in consequence, direct observations 

 of their parallax are not very consistent. In the hope that 

 an investigation of their proper motions mighl give heller 

 results, J. C. K.aplex n has recently <xaminid the extensive 

 series of nebuht made by Miinniclmieyer, ,it I>onn. For 

 convenience and accuracy of calculation, the whole term of 



the proper motion is not used, but only that component of 

 it directed towards tlic aulaijcj-. The analytical method of 

 reduction adopted assumes that the sum of the projections 

 on some determined direction of the peculiar proper motion 

 \anishes in the case of very numerous nebulae; of the 208 

 iiehuht available for the discussion, i68 were finally selected, 

 .uul the mean deduced parallax of these is 0.0046" + 0.0012 ''. 

 It is important to note that this value is very nearly equal 

 to that found for the mean parallax of stars of the tenth 

 magnitude, and as this result is from the discussion of only 

 thirty years' observations, it is suggested that much better 

 ileterminations might possibly result from a photographic 

 investig.ation, which would render possible the measurement 

 and reduction of the places of a much greater number of 

 nebula;. 



Variation of Absorption Bands of Crystals 

 in Magnetic Field. 



.\ll delerminalions of the circumstances causing modifica- 

 tions of spectrum lines of substances, either as to intensity 

 or position, become of great importance in astronomical 

 spectroscopy when applied to the e.xamination of stellar 

 spectra, where many peculiar features are found which have 

 not, as yet, been produced terrestrially. Professor 

 Becquerel has recently given an account of several re- 

 searches, in which he subjected the absorption spectrum 

 of various crystals to very strong magnetic fields, and 

 he finds very interesting changes thereby introduced. The 

 crystal showing the effect most clearly was Zenotinc (a 

 phosphate of yttria, with erbium and rare earths). This 

 is a uniaxial crystal, which exhibits very fine absorption 

 Ijands, which were S])ectroscopically examined with a Row- 

 l.md grating, a nicol prism being' introduced to separate 

 the polarised components. The wave lengths of the lines 

 observed were determined from a comparison spectrum of 

 iron. The effect of the field (which was used up to 

 31,800 C.(j.S.) is much larger than the corresponding 

 action on metallic vapour spectra, and is found to vary 

 with the orientation of the crystal, and the direction and 

 strength of the field. 



Important instances of dissymmetry are mentioned, which 

 appear to be independent of the direction of the magnetic 

 field. Another remarkable feature is the variability of 

 sfiise in which the magnetic field displaces circular vibra- 

 tions, whose sense is originally the same. This appears to be 

 a selective action, which, Professor Becquerel suggests may 

 be due to certain bands corresponding to the vibrations of 

 positive electrons, which, if proved, will add considerably to 

 our knowledge of the inner constitution of matter. — 

 Comptcs Heiiihis, 13-15 (1906). 



New Radcliffe Catalogue of Stars 

 for Epoch 1900. 



The new cil.ilogue reciMitly issued from the R.idclitTe 

 Observatory, O.xford, contains the results of observations 

 made with the transit circle between the years 1894 and 

 1903, both inclusive, under the direction of .\. A. Rambaut. 

 Owing to various causes beyond control, the observations 

 were interrupted several times, but this has not been al- 

 lowed to inliuence the precision of the results, which are 

 shown to be of a high order of accuracy. The present cata- 

 logue gives the jjosition of every star down to the seventh 

 magniludi." contained in the zone 85" — 90° X.P.D., with 

 \ery few exceptions, and those only in the case of double 

 or multiple systems. 



Very elaborate determinations of the pivot errors were 

 made by a nuidification of Hamy's method, in which inter- 

 ference fringes were produced between two plates, one of 

 which suffered displacement when various parts of the 

 pivot were being examined. 



h'ull .ui.ahtical details are given of the reductions, and 

 minii.ii i-iMis with oilur standard catalogues. 



Shadow Bands at Sunrise and Sunset. 



.Another series of interesting observations, which appear 

 to throw light on the curious shadow band phenomena 

 during total .solar eclipses, have been recently described by 

 Monsieur C. Rozet. In December, 1905, M. .\manii saw 

 distinct bars of light and shade on the surface of ;i parti- 

 tion, at the instant of the sun's appearance above a moun- 

 t.iin. This led M. Ro/et to make arrangements for ob- 

 taining a more definite determination, and he fitted up a 



