;54 



NA TURE 



"Au(;i>T 13, 1896 



road construction are given in an instructive and interesting 

 form. To quote the words of Mr. C. W. Dabney, As.sistant 

 Secretary of the Department : " It lias been sought to make the 

 volume a concise reference book of useful agricultural informa- 

 tion based in great jiart U|)on the work of this and other 

 Departments of the Government, without making it an en- 

 cyclopaedia of general information. In brief, the effort has been 

 to make a book, and not a mere Government report — a book 

 worthy to be published in an edition of half a million copies 

 and at an expense to the people, if we count both publication 

 and distribution, of over four hundred thousand dollars." The 

 money thus spent in disseminating accurate knowledge of 

 agricultural investigations may appear excessive, but it will be 

 returned to the country a hundred-fold. 



The additions to the Zoological Society's Gardens during the 

 past week include a Black-faced Kangaroo (Macropus mela)iopiis, 

 i ) from Australia, presented by Mr. G. T. Wills ; a Loder's 

 Gazelle (Gazella loderi, 9 ) from Oued Souf, Algeria, presented 

 by Mr. A. B. Birdwood ; a Gazelle (Gazella ), two Hairy- 

 footed Jerboas [Dipiis liirlipes), a Spot-bellied Snake [Zammis 

 ventrimacitlalus), an Ocellated Sand Skink {Seps ocellatus) from 

 Arabia, presented by Dixon Bey ; a Common Cormorant 

 [Phalacrocorax carho), British, presented by Miss G. Howell ; 

 two Passerine Parrots (Psi/tacula passerina] from South 

 America, presented by Miss L. Scott Moncrieff; a Brown 

 Capuchin [Cebtis fatuelhis) from Guiana, a Grey Ichneumon 

 (Herpestes griseus) from India, deposited ; two Patagonian 

 Cavies (Dolkhotis palachonica), two Vpecaha Rails {Ara/iiides 

 ypecaha), bred in the Gardens. 



OUR ASTRONOMICAL COLUMN. 

 Brooks's Comet. — This comet, which M. Javelle, of Nice, 

 has fortunately re-discovered, remains so faint an object, that 

 other observations for the improvement of the elements, 

 computed by Dr. Bau.schinger, are .still wanting. The one 

 position secured has been utilised to correct the mean motion, 

 and consequently the time of perihelion passage. This will take 

 place November 4'i8375, Berlin mean time, or only 0'2o83 days 

 later than the time determined from the last appearance. The 

 excentricity needs probably a small correction, but the data for 

 its determination are not yet existing. The following ephemeris, 

 for Berlin midnight, is derived from the corrected mean motion 

 and time of perihelion passage. 



1S96. R.A. Decl. Blight 



Aug. I 



Sept. 



For finding the comet, the bright star Fomalhaut will still be 

 convenient, the region comprised in the ephemeris being 

 about 11° north of the star, and on the meridian (London) 

 about 1-45 a.m. 



Meteor Trails.— We noted on July 30 (p. 301) that 

 attention has been called by Prof Johnstone Stoney and others 

 to the desirability of observing the meteors in November next, 

 which are likely to form part of the great November shower, 

 particularly with the view of settling the question of the date at 

 which the shower was introduced into the solar system. 

 Improved methods of observation might have been expected to 

 furni.sh more accurate information, and lead to a closer 

 approximation to the orbit. It is therefore disappointing to 



NO. 1398, VOL. 54] 



read in the Report i.ssued by Dr. KIkIn, the Director of the 

 Vale Observatory, that, notwithstanding repeated efforts, no 

 photographic records of meteor trails have been secured. The 

 apparatus was in u.se fcjr the .-Vugust meteors, but none were of 

 sufticienl brilliancy to impress themselves upon the film, which 

 had become .somewhat fagged by the strong moonlight. Other 

 occasicms were equally disapi^oinling : but the Director is not 

 discouraged, and in |.)lace of the two lenses now employed he 

 hopes to substitute the complete battery of lenses for which the 

 mounting was originally planned. 



Personai, Equation in Observing Transits. — The 

 vexed question of the existence and necessary removal of 

 personal equation in determining clock error has been attacked 

 by Mr. R. H. Tucker, of the Lick Observatory. The particular 

 form of the question to which Mr. Tucker has applied himself is 

 that raised some years since by Prof, van der Bakhuyzen, of the 

 effect of the brilliancy of the star on the time of transit 

 determined by chronographic registration. Mr. Tucker placed 

 over the object-glass four thicknesses of wire netting, which 

 reduced the magnitude of the star 4-1 magnitudes, or, in other , 



words, destroyed all but one forty-fifth part of the original light. m 



The clock error was determined from the observations of stars, I 



with and without the screen alternately, with the result that the 1 



faint stars were observed 0-037S. later than when seen at their 

 full brilliancy. The correction to observed right ascension is 

 -0'009s. for each magnitude, with a probable error of +o-oois. 



RECENT RESEARCHES ON RONTGEN RA VS. 

 T^HE subjoined summary brings together in a convenient form 

 ■*• for reference a number of researches on Rontgen rays 

 which have recently come under our notice. It will be seen 

 that a large amount of detailed information with reference to 

 the character and capabilities of the rays is being accumulated 

 by investigators in various parts of the world. 



Dr. A. Dupre, F.R.S., writes, under date July 29 : 

 " The article by Mr. Benjamin Davies, in your issue of July 23, 

 has recalled to my mind certain experiments of my own, made 

 several months since, which may perhaps throw some light on 

 Mr. Davies' results. I was then working with various vacuum 

 tubes, and among others with an ordinary Geissler tvibe containing 

 nitrogen, such as is used for obtaining spectra of gases. The 

 capillary part of this tube gave a brilliant light, which had the 

 power of inducing fluorescence of many substances, to a remark- 

 able degree, the light falling direct on to the subst.ance. The 

 tube being in action, the screen covered with platino-cyanide of 

 potassium fluoresced strongly ten feet from the tube, the active 

 surface being towards the tube. This was, of course, to lie 

 expected, but, to my astonishment, the fluorescence was almost 

 equally noticeable when the back of the screen was turned 

 towards the tube, and remained so even when I interposed a 

 book, a board, a sheet of tin-plate, or the human body between 

 the tube and the screen. When, however, I placed my hand 

 against the back of the screen, no trace of a shadow was 

 noticeable ; the .same was the case when pieces of metal, or other 

 objects opaque to the Rontgen rays were so placed. The screen 

 all the while remaining strongly and uniformly fluorescent. 

 This seemed to me to show that, whatever the nature of the 

 rays producing the fluorescence of the screen, they could not be 

 Rontgen rays ; and I concluded that the fluorescence was really 

 due to light striking the front, or active, surface of the screen 

 after reflection, either from the walls of the room, or, perhaps, 

 from the air. When accordingly all possibility of any light thus 

 reaching the screen was excluded, all fluorescence was eft'ectually 

 stopped. Might it not be possible that in Mr. Davies' experi- 

 ment the fluorescence of his screen was in part, at least, induced 

 by rays reaching the active surface of the screen after reflection ? 

 Thus accounting for the fact that the hand cast no shadow 

 whatever." 



Mr. J. A. M'Clelland read a paper on the "Selective Ab- 

 sorption of Rontgen Rays" before the Royal Society on June 

 18. The experiments described in the paper were made to 

 determine whether or not the Rontgen rays given off by a vacuimi 

 bulb were of a homogeneous nature, by examining the manner 

 in which they are absorbed by different substances. The sub- 

 stance whose absorptive power was to be examined — say, a jilale 

 of glass — was placed .so that the rays traversed it before falling 

 on a charged disc, which was in connection with a jiair of 



