May 12, 1892] 



.VA TURE 



They burn with a blue flame upon ignition, producing water 

 vapour, carbon dioxide, and hydrofluoric acid. They possess 

 an odour somewhat resembling that of carbonyl chloride. Water 

 dissolves about twenty times its volume of the gas, but the liquid 

 does not mix with water, a very small proportion only being 

 dissolved, and suflfering slow decomposition. Alcohol, ether, 

 benzene, and chloroform dissolve it in all proportions. 



The additions to the Zoological Society's Gardens during 

 the past week include a Rhesus Monkey (Macacus rhesus i) 

 from India, presented by Mr. C. Drew ; a Grivet Monkey 

 {Cercopitheats gn'seo-7'indis 9 ) from North-east Africa, pre- 

 sented by Mr. George Conquest ; a Grey Ichneumon {Herpestes 

 griseus), from India, presented by Mr. J. E. Barber ; a Com- 

 mon Fox [Cariis vulpes ? ), British, presented by Miss Nora 

 Dunn ; a Song Thrush { Turdus musicus), British, presented by 

 Mr. Baldwin M. Smith; an Alexandrine Parakeet {Palaarnis 

 alexaiidri i ) from India, presented by Mr. E. Bond ; two 

 Cerastes Vipers (Vipera cerastes) from Egypt, presented by 



Colonel Holled Smith ; a Lizard {Amphibolurus sp. inc., 



from Australia, presented by Mr. Herbert E. Swayne ; a 

 Guinea Baboin {Cynocephalus sphinx ? ) from West Africa, a 

 khesus Monkey (Macacus rhesus <J ), a Grey Ichneumon {Her- 

 pestes griseus) irom India, two Punctated Agoutis {Dasyprocta 

 punctata), a King YuXiMre {Gypagus papa)hom Central America, 

 a White-eyebrowed Guan {Penelope superciliaris) from South- 

 east Brazil, deposited ; a White-faced Heron {Ardca nova- 

 hollandi(E) from Australia, eight Ruffs {Machetes ptignax 4 {, 

 4 ? ), British, purchased ; a Reindeer {Rangifer tarandus ? ) 

 born in the Gardens. 



OUR ASTRONOMICAL COLUMN. 



Photographic and Visual Magnitudes of Stars.— 

 At the Amsterdam Academy of Sciences on April 2, Prof. J. C. 

 Kapteyn communicated the results of an investigation on the 

 systematic differences between the photographic and visual 

 magnitudes of stars in different regions of the sky. The com- 

 parison of the photographic diameters of stars of equal visual 

 magnitude (according to Gould and Schonfeld's estimations) on 

 370 plates of the southern sky, shows that the actinic effect of 

 stars in or near the Milky Way is much greater than that of 

 stars in high galactic latitudes. Prof. Kapteyn has examined 

 the different causes which lead to this variation. There is, first 

 of all, the influence of different meteorological conditions ; 

 next, systematic errors in the catalogue of visual magnitudes 

 used for comparison ; and thirdly, peculiarities in the light of 

 the stars. The discussion leads to the conclu^^ion that the 

 difference of magnitude is not appreciably affected by the first 

 of the.se causes. And since, taking everything into con- 

 sideration, the errors of estimated visual magnitudes could not 

 possibly exceed 0-3 magnitude, there is no doubt that the differ- 

 ence of half a magnitude or more, indicated by the photographs, 

 is due to the quality of light emitted. It is said that Prof. 

 Pickering's idea that the Milky Way ought to be considered as an 

 aggregation of stars of the first type is only sufficient to account 

 for a difference of about O'l magnitude. Thus it appears that 

 the light of stars in or near the Milky Way, like those of 

 Group IV., is richer in violet rays than that of other stars. 



Photographs of the Lyra Ring Nebula.— In addition 

 to the work on the Carte du Ciel, Prof. Denza, of the Vatican 

 Observatory, has taken up the photography of nebula-. Be- 

 ginning with the Ring Nebula in Lyra, he has made five ex- 

 posures on this object, from half an hour up to nearly two hours 

 duration. To bring out the fine detail, development has 

 been carried on for about twenty minutes in each case. The 

 negative which had received the longest exposure was pre- 

 sented to the Paris Acidemy on April 25. Viewed microscopic- 

 ally, the star at the centre of the nebula is seen to be joined 

 to a smaller one near the nebulosity, and each of them can be 

 broken up into other more or less brilliant points. A large 

 number of condensed regions are well visible in the nebula. 

 The location of these leads Prof. Denza to agree with Secchi that 



NO. II 76, VOL. 46] 



" L'anneau se prolonge dans lesens du plus grand axe, et one 

 les parties les plus denies sont dans la direction du petit axe. 



Detekminatio.v of the Constant of Aberration, — 

 Prof. G. C. Comslock contributes the provisional results of a 

 determination of the constant of aberration to the Astronomical 

 yournal, No. 261. The method adopted in the investigation is 

 a modified form of that used by M, L<Twy, three reflecting 

 surfaces being placed in front of the objective of the telescope 

 instead of two. Images of stars in different portions of the 

 heavens are thus simultaneously produced in the focal plane of 

 the objective, and a micrometer is used to measure the distance 

 between those of two given stars, when each pair of surfaces is 

 successively employed. Then, if d represent the distance 

 between the images of two stars as measured with the micro- 

 meter, A the angle subtended at the earth by the stars, R the 

 effect of refraction in changing the true A into an apparent A', 

 and K a correction depending upon the squares of the errors of 

 adjustment of the mirrors, we have — 



A = 120' -t- K -f </ 4- R. 

 The provisional value of the aberration constant derived from 

 Prof. Comstock's observations is — 



2o"*494 ± o"'oi7. 

 An investigation of the refraction has also been made, resulting 

 in the detection of a real variation. The refraction is at a 

 maximum near the time of the winter solstice and a minimum 

 near the summer solstice, but the exact epoch and amplitude 

 have not yet been det . mined. 



Star Magnitudes. — "The Estimation of Star Magnitudes 

 by Extinction with the Wedge," was the subject of an interest- 

 ing paper by Captain Abney before the Royal Astronomical 

 Society, many of the experiments from which his conclusions 

 were drawn being made from a paper which he and General 

 Festing communicated to the Royal Society on colour photo- 

 metry. In the experiment for determining the amount by which 

 the intensity of any ray of the spectrum would have to be re- 

 duced before it became invisible, the absolute intensity of the D 

 line was fixed upon for the basis, from which all the other in- 

 tensities could be directly calculated. With the arrangement he 

 described, the D line was reduced to the 350 ten-millionths 

 part of a standard amyl lamp, while under the same conditions 

 the green light E had to be reduced to 65, F to 150, G to 3000, 

 and the red to 110,000 ten-millionth part. By making the rays 

 equal to one amyl lamp the numbers obtained were for D 350, E 

 35, F 17, G 15, and for C 22,000 ten-millionths part. 



These numbers showed that to produce extinction for 

 two lights of equal luminosity, say of colours C and G re- 

 spectively, the latter was nearly 1500 times greater than that 

 required for the other. He then referred to the extreme persist- 

 ency of the violet sensations, they being 1 500 times more persistent 

 than the red and about 25 times more than the green, pointing 

 out that the violet sensation would (je the last to be extinguished. 

 The function of the wedge, then, was not to obliterate the 

 spectrum but to eliminate the violet sens-ation contained in its 

 light. By determining star magnitudes by this method of ex- 

 tinction, the results obtained, he says, " should agree better with 

 those obtained by photography than those obtained by eye 

 estimation," the first being obtained by estimation of the E light, 

 the second of the light between G and F, and the third from 

 that near D. 



Referring to colour extinction he mentions that although most 

 of the faint stars are known to be of a bluish colour it does not 

 follow that "they are not red." The blue tint is brought about 

 by the faintness of the light, which makes all colours appear grey, 

 and " as the violet sensation disappears last, it frequently 

 happens that you get the red and green sensations as grey, and 

 the violet just above the cjlour limit, thus giving a grey blue." 

 He suggests that with telescopes of large aperture these stars 

 might be seen in colours. 



THE INSTITUTION OF MECHANICAL 

 ENGINEERS. 



AN ordinary general meeting of the Institution of Mechanical 

 Engineers was held on the evenings of Thursday and 

 Friday of last week. There were two items of exceptional 

 interest on the programme, the first being the inaugural address 



