J™e 1, 18M.] 



KNOWLEDGE 



135 



East Indies in the same season. Some of the seeds were 

 sent to Barbadoes and there planted, with extraordinary 

 results, hardly any two of the plants produced being alike. 

 Between some the differences were so great as to make 

 them hardly recognizable as the same plant ; one in parti- 

 cular was remarkable in having long roots springing from 

 every joint in the stem. It is thought that many new and 

 improved varieties might be produced if these plants could 

 be induced to seed regularly. 



Some beautiful photographs in colours of the solar 

 spectrum have been taken by the improved method of M. 

 Lumiere. The essential principle of this colour photography 

 is as follows : A sensitive film spread in the usual way over a 

 glass plate is laid upon mercury, film downwards. Upon 

 exposure the light rays fall through the glass upon the 

 film, penetrate the film, and are reflected back through it 

 from the surface of the mercury. The reflected vibrations 

 interfere with the direct waves, so that at intervals the 

 vibrations are neutralized, and at intermediate points they 

 are intensified. At the former levels there will evidently 

 be no photographic effect, while at the latter there will be 

 a maximum. On developing, therefore, the part of the 

 film acted upon by light of any colour, it is found to be, as 

 it were, stratified, the strata being at a distance from each 

 other equal to half the wave-length of the light in question ; 

 consequently when viewed by reflected light, the same 

 colour is produced by interference. 



Hetttrs. 



[The Editor does not hold himself responuible for ths opinions or 

 statements of correspondents.] 



SiK, — Will you kindly inform me what part of our 

 planet the Leonids are likely to strike on the 14th 

 November, 1899 ? WlU England be a good vantage ground 

 for seeing them ? and will there be a moon ? 



I am, yours faithfully, 

 Litcham, Norfolk. A. Collison. 



[The rich part of the Leonid stream of meteors must 

 be several hundred thousand miles in diameter, for in 1833 

 and 1866 the glorious display of meteors lasted for many 

 hours. Since the earth in its orbital motion around the 

 sun only takes about seven minutes to traverse a space equal 

 to its own diameter, it follows that on the above occasions 

 the earth must have been entirely immersed in the rich 

 part of the Leonid stream, and that all parts of the earth's 

 atmosphere on the hemisphere turned towards the Leonid 

 radiant must have been peppered by meteoric particles ; 

 the central portions of the hemisphere, no doubt, receiving 

 a greater share of bombardment, area for area, than the 

 regions near to the limb. The Leonid radiant is situated 

 at E.A. 150^ and N. Dec. 28°. The longitude of the part 

 of the earth's northern hemisphere which will meet the 

 greatest number of meteors will depend upon the exact 

 time at which the earth passes through the thickest region 

 of the swarm. The striking character of the display will, 

 no doubt, be diminished by the light of the moon, which 

 will be full on the 18th November, 1899. Those who wish 

 to avoid the chances of November fog and cloud during 

 the display would, no doubt, do well to go to Algeria or 

 Egypt, where the radiant will pass near the zenith. — 

 A. C. Rantabd. t^^ 



VISIBILITY OF THE DARK LIMB OF VENUS. 

 To the Editor of Knowledge. 



Sir, — In connection with the letter by M. Kudaux, in 

 this month's Knowledge, I should like to be permitted to 



reiterate what I have on several occasions previously 

 stated, both orally and in print, viz., that whenever Venus 

 has been favourably situated, in or near her inferior 

 conjunction, I have never failed to detect her unilluminated 

 limb — in fact, that it has been so very visible as to have 

 been at once perceived by totally inexperienced observers 

 to whom I have been showing the planet. 



But there has been no doubt whatever as to its appearing 

 distinctly darker than the surrounding sky, and my own 

 idea has always been that this had its origin (1) in the 

 fact that it is the refraction of the sun's rays through the 

 atmosphere of the planet, which, as it were, gives a very 

 delicate bright fringe or seeming background to it ; and 

 (2) gives thus a complete disc, very slightly darker than 

 the sky on which it is projected, as an effect of contrast to 

 the brilliant hair-like illuminated crescent which partly 

 surrounds it. 



I do not believe in the unilluminated part of Venus 

 being ever really seen as brighter than the sky, save as a 

 purely optical illusion. 1 have myself, on several occasions, 

 been able apparently to trace her entire outline thus, in 

 the twilight ; the dark portion presenting substantially the 

 appearance of the lumiere cendree, or " old moon in the 

 new moon's arms," with which everyone, astronomer or 

 not, is so familiar. But on occulting the bright limb of 

 the planet by suitable means, the pseudo-dark limb has 

 absolutely vanished ; showing conclusively, to my mind, 

 that it had its origin somehow or somewhere in the 

 eyepiece. Yours faithfully, 



Forest Lodge, Maresfield, Uckfield, William Noble. 

 May 9th, 1894. 



COMET I. 1894. (DEXNINa).— PERIODICAL COMETS. 

 To the Editor af Knowledge. 



Slr, — The very small inclination (6^-°) computed for this 

 comet a few days after its discovery induced the belief 

 that it was revolving in an ellipse of short period, and this 

 has now been reahzed, for M. Sehulhof, in Astronomisclie 

 Nachrichten, No. 8227, gives elliptic elements corresponding 

 to a periodic time of 6745 years. A more exact de- 

 termination may possibly be made should additional 

 observations be forthcoming, for it is highly probable the 

 comet is still observable in a few of the largest telescopes. 



If the computed period of six and three-quarter years is 

 correct, it would appear that the comet is only perceptible 

 to us once in four returns, i.e., every twenty-seven years, 

 for at other times its position is too far from the earth. 

 In 1867, if the orbit were the same as at present, the comet 

 might have been well seen, for the earth was nearly in 

 the same longitude as the comet's perihelion at the time 

 the comet was there, and so the distance separating the 

 two bodies must have been near the minimum. This 

 ought to occur again in February, 1921, as the comet and 

 earth will be on the same side of the sun, and the former 

 will be projected on much the same region of sky as during 

 its recent apparition. It is remarkable that though the 

 comet was favourably visible and at a considerable altitude 

 in December, 1893, January and February, 1894, it was 

 not discovered until the end of March. The fact is 

 significant as proving the search for these bodies is not 

 nearly so rigorous as it might be. 



M. Sehulhof expresses the belief that additional 

 periodical comets await discovery, and that a promising 

 field for the search will be iu that region of the sky opposite 

 to the sun. The experience of recent years fully bears 

 out the opinion of the eminent authority named, for the 

 number of short-period comets added to the list is quite 

 surprising : — 



