64 



NA TURE 



[November 17, 1892 



The motion of this comet will be noticed from the above 

 ephemeris to be very rapid in a southerly direction, amounting 

 to about 1° per day. 



The Light of Planets. — The question as to whether the 

 light of planets is capable of casting shadows must have, 

 especially during the last few months, been in the minds of 

 many, and perhaps many observations have already been made, 

 but unfortunately not published. With regard to this question, 

 V Astronomic for November contains tv\o notes, the first of 

 which, communicated by M. Marcel Moye on August 30, 

 relates to the planet Mars. His observations were made just 

 before the meridian passage and in a room where the light of 

 the planet could enter the open window. In this way white 

 paper invisible in the corners of the room was easily dis- 

 tinguished when placed on the wall opposite the window, while 

 one could see well the shadows between the fingers of the hand ; 

 placing a newspaper in the light of Mars only the place of the 

 table and the number of the words could be recognized, but not 

 read, as was the case with Jupiter. M. Moye concludes then 

 that Mars certainly casts shadows, less strong than those of 

 Jupiter but still appreciable. 



In the note on the light of Venus M. Leon Guiot tells us 

 that on August 29, when about to get up to observe Jupiter, he 

 was astonished at the brilliancy ot the light that entered his 

 window. Observing his watch, which was hanging on the wall, 

 he was actually able to trace its shadow on the wall, for he says 

 that all was visible as in the light of the moon ; one could even 

 read the newspaper. It was about this time that Venus was 

 constantly seen with the naked eye in full daylight. 



Stellar Magnitudes in Relation to the Milky Way. 

 — Prof. Kapieyn is the author of an important memoir, which is 

 published in the Bulletin du Coinite International Permanent 

 pour I'execution pholographique de la carte du ciel, relative to 

 an observed systematic difference between the photographic and 

 visual magnitudes of stars depending on their distance from the 

 Milky Way. Prof. Kapteyn first noticed that a difference 

 existed in 1890, but in the present paper he presents a pre- 

 liminary account of the results he has obtained. The cliches 

 which have been under examination were exclusively those 

 made at the Cape Observatory for the chart mentioned above. 

 In this discussion he has adopted the two following laws : 

 (i) that increasing the time of exposure in the proportion of 

 I to 2 "5, the fraction of a magnitude gained is 07, and (2) the 

 atmospheric extinction of actinic rays rises to 2| above the 

 visual rays. Since there is an undoubted difference between the 

 photographic and visual magnitudes, denoting this difference by 

 the symbol Aw, the author commences to investigate whether this 

 quantity is ever equal to zero, that is when the photographic 

 and visual magnitudes are equal, and if so to find the locus of 

 these points. Charting the points down on a map and con- 

 necting them up by means of curves, the latter are found to 

 follow in a striking manner the path of the Milky Way. Table 

 II. gives the values of Am obtained from several cliches, and 

 the positive values lie without exception between these two 

 curves, while the negative ones are situated without. Taking 

 into consideration both bright and faint stars, that is stars from 

 the 4th to the loth magnitude, the author finds that there 

 a strong relation depending on their galalactic latitude exists 

 between them, whether they be even very near or distant from 

 the Milky Way, and the same systematic variation of Aw 

 apparently holds good, being represented by the formula 



Am = a + K0 



representing the galalactic latitude and /c = - o""oo99 ± 



In seeking for an explanation of the difference, Prof. Kapteyn 

 investigates each possible cause singly. His conclusion, to 

 state briefly, amounts to this, that, if one considers the stellar 

 magnitudes given in the " Uranometrique " and in Gould's 

 " Catalogue of Zones" (it is from these two sources that he has 

 obtained the visual magnitudes) to be correct and not subject to 

 systematic errors amounting sometimes to as much as half a 

 magnitude, then it must be concluded that the light of the stars 

 situated in the Milky Way or in its vicinity is much richer in 

 aclinic rays than those at considerable galalactic latitudes. We 

 may remark that the publication of this paper has been purposely 



NO. 1203, VOL. 47] 



hurried owing to the importance of the matter therein, but 

 although sufficient observations have not been taken in account 

 for a very rigid investigation, Prof. Kapteyn hopes to eli ninate 

 many of the difficulties and accidental errors by the discussion 

 of cliches of different regions of the sky, differing in galalactic 

 latitudes, made at equal altitudes, on similar plates, with equal 

 lengths of exposure. 



The Canals of Mars. — The late opposition of Mars, and 

 the re-observation of the doubling of the canals has brought 

 forward many theories relative to this very curious phenomenon. 

 There seems to be no doubt now that this doubling is not due 

 to instrumental deficiencies, or even to an optical delusion 

 caused by the fatigue of the eyes ; but that it is a real observed 

 fact and therefore requires a rigid explanation. Omitting the 

 now well-known hyp )theses suggested up till quite lately, the 

 most recent is that put forward by M. Norman Lockyer and 

 which is recorded in Nature, vol. xlvi. p. 448. Mr. Lebour 

 also (Nature, vol. xlvi. p. 611) points out the likeness of these 

 markings to the cracks produced in glass broken by torsion, 

 adding that the chief characteristic features in the Mars' lines 

 are there produced. In Comptes rendus (No. 18) for October 31 

 M. Stanislas Meunier relates another possible cause, and illus- 

 trates the phenomenon experimentally. The exjieriment is 

 as follows : — He takes a polished metallic surface and on it 

 traces a series of lines and spots, representing as nearly as 

 possible the Martial surface as seen by us, and illuminates it all 

 by sunlight. He then stretches at some distance (a few milli- 

 metres) from it a fine transparent piece of muslin. Looking 

 at the surface through this medium he finds that all the lines 

 and spots are doubled, and, " se germiner par suite de ra()pari- 

 tion, a cote de chacune d'elles, de son ombre, dessinee sur la 

 mtiusseline par la lumiere que le metal a reflechie." A fact 

 observed by M. Schiaparelli is that the canals when doubled 

 are not always exactly parallel, and that sometimes there is an 

 "aspect de nebulosite." These and other peculiarities are, 

 according to M. Meunier, reproduced by simply undulating 

 the muslin. His explanation is that the solar light is reflected 

 from the planet's surface very uneqnally, that from the con- 

 tinents exceeding that emitted by the deeper parts, seas and 

 canals. Although the atmosphere is a limpid one, we are 

 unable to see its motions ; but if, as he says, the serial envelope 

 includes a transparent veil of fog at a suitable height, a contrast 

 would be produced, as was the case with the muslin, by the 

 production of shadows " qui pour une ceil place ailleurs que sur 

 le prolongement des rayons reflechis, a cote de chacune des 

 surfaces peu reflechissantes, une image pareille a elle." This 

 explanation of the phenomena of shades by reflection if valid 

 should of course hold good for the planet Venus when properly 

 situated, and that it is not observed on the Moon is only 

 another proof that our satellite has no atmosphere. 



GEOGRAPHICAL NOTES. 



The Revue Francaise states that a subterranean town, laid 

 out with regular streets in a series of great caverns, near Karki, 

 on the right bank of the Amu-daria, has recently been explored. 

 Pottery and metal work were found amongst the ruins, and from 

 the coins and inscriptions seen the town must have been 

 occupied at least as early as the second century B.C. 



By the new constitution of the United States of Brazil the 

 seat of government is to be transferred from Rio de Janeiro to 

 a site upon the central plateau where an area is to be marked 

 off as a federal district. A scientific mission under Senor 

 Cruls has been appointed to examine the region where the three 

 rivers, Sao Francisco, Tocantins, and Parana, take their rise at 

 an elevation of over 3,000 feet, with the view of finding a suit- 

 able site for the new capital. 



Mr. D. J. Rankin communicates to the i't^/Zw/^ Geographical 

 Magazine an account of his journey up the Zambesi in 1890-91, 

 with a map of the country between the Zambesi and Shire. 

 He found the Zambesi freely navigable for light-draught 

 steamers as far as the Acababassa Falls, more than 300 miles 

 from the sea, the Lupata narrows presenting no difficulty. 

 Between Lupata and Acababassa extensive coal deposits occur, 

 and these are sure to become valuable. Beyond the falls after 

 a portage of about thirty miles, the Zambesi is again navigable 



