January 7, 1892] 



NATURE 



237 



OUR ASTRONOMICAL COLUMN. 



Wolf's Periodic Comet. —The following ephemeris, due 

 to Dr. Berberich, is from one contained in Edinburgh Circular 

 No. 21 : — 



Brightness, that 

 '892- A.;r*;,"„ Declination. at discovery 



Jan. 6 ... 4 15 15 ••■ - 13 39 5 ••■ 3'24 



13 9 8 ... 2-93 



12 36 9 ... 2-66 



12 I 4 ... 2*41 



II 23 8 ... 2-19 



10 44 7 ... 1-99 



10 4 5 ... i-8i 



Feb. '3 ■• 31 12 ... 9 23 7 ... 1-65 



8 42 6 ... i'5i 



8 I 6 ... 1-38 



7 20 9 ... 1*26 



6 40 8 ... 1*15 



6 I 5 ... i'o6 



. __ 5 23 2 ... 0-97 



March 2 ... 1; I 30 ... 4 46 o ... q- 



4 10 I ... 0'82 

 3 35 5 ••• 076 

 3 2 4 ... 070 

 2 30 8 ... 065 

 209... o*6o 

 I 32 7 ... 056 

 160... 0-52 

 The comet is now a comparatively faint object, but can be 

 seen with instruments of moderate aperture. It is in Eridanus, 

 and moving slowly towards Orion. It has the same declination 

 as Rigel on February 9, and passes about 4'' north of this star 

 on March 7. 



The Diffraction Effects produced by placing 

 Screens in front of Object Glasses formed the subject 

 of a communication by Prof. Pritchard to the Paris Academy 

 on December 28. The paper deals with the effects produced 

 by meshes of metallic wires on the photographic and visual 

 images of stars in the focus of refractors (photographic and ordi- 

 nary) and reflectors. Numerous screens have been used in the 

 investigation, but only the results obtained with three, designated 

 by A, B, and C, are described. A was an iron wire screen 

 having very nearly square meshes about 09 mm. apart, B a 

 bronze one with meshes having an area of about 0*225 square 

 mm., and C one having circular holes about 2*3 mm. diameter 

 in it. Photometric observations of the intensities of the visual 

 images of a bright star with and without A gave the relation 

 1/3 636 which, expressed in units of stellar magnitude of the 

 ordinary scale, corresponds to an absorption by the screen of 

 I '40 magnitude. The same screen placed in front of a photo- 

 graphic telescope reduced stars of the ninth to the eleventh 

 magnitude. There was thus a variation of |- of a magnitude in 

 the effect produced by the same screen on the visual and photo- 

 graphic images of the same star. B intercepted light equivalent 

 to 24 magnitude in the case of the observing telescope, and 28 

 magnitude in the case of the photographic one. C gave the 

 values I "44 and I 83 magnitudes respectively. In all cases, 

 therefore, the absorption of light was greater for the photographic 

 than for the visual image. This is most probably due to the 

 different treatment of the lenses by the optician in correcting 

 them for photographic or visual work. Using the same 

 screens in connection with reflectors, the intensities of the visual 

 and photographic images were found to be the same, and the 

 quotient of the intensity without a screen into the intensity with 

 a screen was very nearly equal to the square of the portion of 

 the screen traversed by the light. 



Refr.active Power of Cometarv Matter. — Mr. Bar- 

 nard communicates a paper to the Astronomische Nachrichten, 

 No. 3072, on observations made of the difference of declination 

 of 21 Asterope and 22 Asterope at the passage of Wolf's comet, 

 1891 September 3, through the Pleiades. As the opportunity 

 afforded a good test to determine the refractive power of 

 cometary matter, he, together with Mr. Burnham, instituted 

 a series of measures of the declination of these stars before, 

 during, and after, the transit of the comet over Asterope. Mr. 

 Burnham's observations were made with the filar micrometer of 

 the 36-inch equatorial, while Mr. Barnard used that of the 



NO. II 58. VOL. 45] 



12-inch. The results obtained by the former indicated a smal 1 

 change, so small indeed that Mr. Barnard with his instrument 

 could not detect any variation, or at any rale any difference of 

 reading that would not be masked in the errors of observation. 

 Mr. Burnham's measures are shown in the form of a curve, a 

 vertical line corresponding to the time of nearest approach of the 

 nucleus to Asterope, while a horizontal one indicates the mean 

 a5. The dots representing the observations gradually increase 

 their declination, and then more quickly decrease, rising again 

 only when the comet has transited the star. The results are 

 most interesting, and the following is a short extract from the 

 table. The time of nearest approach of the star and the nucleus 

 occurred at 2h. 87m. sidereal time. 



Difference of Declination between 21 Asterope and 22 Asterope. 



"monthly" contains an interesting article by Dr. W. Zenker 

 on " The Heating of the Earth's Surface by the Sun." That on 

 " The Great Ice Age," by Prof Dr. Albrecht Penck, begun in 

 the November number, is here completed. 



"Washington Observations, 1886." — This volume, just 

 issued, contains the results of all the observations made during 

 the year 1886 at the United States Naval Observatory, under 

 the superintendence of Commander A. D. Brown, U.S.N. 

 Under the heading of " The Transit Circle " and "The Meri- 

 dian Transit Instrument," are given descriptions of the instru- 

 ments, catalogues of miscellaneous stars observed, constants 

 used in reductions, adopted corrections, clock rates and cor- 

 rections, positions and semi-diameters of sun, moon, and 

 planets, and many other details. Under "Observations and 

 Results " are tabulated all the results obtained by the use of 

 the transit circle, meridian circle, the equatorials of 26 and 9 "6 

 inch aperture, &c. The magnetic observations made during 

 the years 1888 and 1889 at the same Observatory by Ensign 

 J. A. Hoogewerff, U.S.N., under the superintendence of 

 Captain F. V. McNair, U.S. N., are also given, including both 

 tabular statements and fourteen large plates. 



MOLECULAR WEIGHT OF GADOLINIA. 

 TN Bihang till K. Svenska Vet.-Akad. Handlingar, Band 17, 

 Afd. ii.. No. I, Prof. A. E. Nordenskiold returns to a subject 

 which will always have a peculiarly fascinating interest for 

 chemists. In spite of the vast amount of time and labour which 

 has been expended upon the investigation of the rare earths 

 contained in such minerals as gadolinite and samarskite, the 

 mystery involved in the peculiar nature and reciprocal relations 

 of these bodies is still far from having been solved. 



It was in 1794 that Gadolin first examined the mineral gado- 

 linite. Since that time the exceedingly complex nature of the 

 new earth which he discovered has been continually demon- 

 strated. Every fresh investigation seemed to result in the 

 announcement of the discovery of a new element. The task of 

 separating these different earths, which are so closely allied in 

 their chemical properties, was exceedingly difficult and tedious. 

 Mosander was one of the first to make use of the process of 

 fractional precipitation, which, combined with spectroscopic 

 methods, has in the hands of such scientific men as Delafontaine, 

 Marignac, Lecoq de Boisbaudran, and Crookes, led to such 

 interesting results. The investigations of Crookes on these rare 

 earths resulted in the famous lecture on the "Genesis of the 

 Elements," delivered before the Royal Institution in 1887. 



The mention of this remarkable lecture brings us to the 

 subject of the present paper, for one of the arguments employed 

 by Crookes was that we had proof of the existence in nature of 

 a mixture of isomorphous bodies, always associated together, 

 and presenting a molecular weight so constant that they almost 

 constitute a single body. That statement was made by giving 



