May, 1915. 



KNOWLEDGE. 



149 



Nova (2) Geminorum. two years after. The chief point is 

 that the nebular lines at 4364, 4960, 5007 have completely 

 disappeared in the two older Novae ; the last two are still 

 strong in Nova Lacertae, while all three are strong in Nova 

 Geminorum. Hence the fading of these lines is characteristic 

 of the late stage of Novae. At the same time, the continuous 

 spectrum grows stronger. They suggest, in explanation, 

 that the initial outburst is due to the star entering a nebula, 

 and that the nebular lines disappear when the star emerges 

 from it. There is an obvious difficulty in this suggestion. 

 It would need a fairly dense nebular to account for the very 

 sudden outburst of light : such a nebula could hardly fail 

 to be a violent exciting agent during the whole of the star's 

 passage through it. But the above Novae had become verv 

 faint before the nebular lines faded out. 



The paper points out that the late stages of Nova spectra 

 are almost the same as those of Wolf-Rayet stars. It is 

 suggested that at least some of the latter stars may have 

 once been Novae. 



THE COLLISION HYPOTHESIS OF NOVAE.— Mr. 

 !■'. A. Lindemann has an article on this subject in 

 Monthly Xotices for January. Examining whether collision 

 with a nebula would explain the observed phenomena, he 

 finds that the pressure of the nebula would have to be 

 about 1 1000 of a millimetre — Mr. Ranyard had shown that 

 so high a density is improbable. The objection is also made 

 that the temperature would increase,, or at least remain 

 constant, all the time that the star was traversing the 

 nebula ; but in fact the decline generally sets in very soon. 

 The hypothesis of collision with a meteor swarm is more 

 hopeful ; but the author thinks that in this case it would 

 return rapidly to its condition before the outburst, whereas 

 the effects on the spectrum persist for many years. He 

 next deals with the assumption that novae arise from the 

 collision of star with star. Making the best assumptions 

 in his power for the various constants involved, he deduces 

 that the number of dark stars in our system must be about 

 four thousand times the number of bright ones. If this 

 be correct, stars remain dark for a much longer period than 

 they are luminous. The period of luminosity of a star is 

 supposed to be measured by hundreds of millions of years, 

 and its dark period would be measured bv billions. Such 

 matters must always remain highly speculative ; still, 

 they have a fascination for manv minds, and the time 

 spent on them is not wasted. 



THE SOLAR ECLIPSE OF LAST AUGUST. - 

 Monthly Notices for January contains the reports of the 

 observers, with two photographs and a careful drawing, 

 which shows that the corona was of slightly modified mini- 

 mum form, differing in the fact that the equatorial streamers 

 were more spread out than at minimum. Father Cortie 

 obtained a successful photograph of the coronal spectrum. 

 Five series of flutings were shown, three in the red and two 

 in the green ; also several lines, including the characteristic 

 line in the green at 5303. The sources of the other lines 

 have not yet been identified. The photographs at Minsk 

 failed to show the 5303 line, and indicated that the violet 

 end of the coronal spectrum was unusually faint. The 

 spectrum of the chromosphere was photographed down 

 to wave-length 3118 in the ultra-violet. Hoth the 

 Minsk observers and Father Cortie took an iron comparison 

 spectrum on their plates, so that accurate measures of wave- 

 length can be made. On the whole, considering the dis- 

 turbance due to the war, the observation of the eclipse was 

 successful ; some of the Russian, French, and Swedish 

 observers also obtained good results. In particular they 

 discovered a new bright line in the red part of the coronal 

 spectrum at wave-length 6374 : this is included in one oi 

 Father Cortie's bands. 



THE ANNUAL REPORT OF THE ROYAL ASTRO- 

 NOMICAL SOCIETY contains several interesting notes on 

 the progress of astronomy. Controversy over the solar 

 constant of radiation continues. Messrs. Abbot, Fowle. 



A. K. Angstrom, and E. H. Kennard support the low value 

 of 1 -93 calories per cm. 2 sec. , while F. W. Very maintains 

 that it cannot be less than 3, and may reach 4. He 

 considers that much heat is reflected from the upper air 

 without reaching the Earth. 



F. Schlesinger gives the formula 14 = -17 — 3 3 -4 sin 2 ^> for 

 the daily solar rotation, being the latitude. This is 

 deduced from spectroscopic measures in the regions of the 

 spectrum 4059 to 4126, and 4220 to 4277. 



The solar activity is now definitely on the up grade, and 

 equatorial spots have died out, showing the demise of the 

 old cycle. 



Mr. D'Esterre has continued his photographic and visual 

 patrol on selected regions of the Milky Way, and has dis- 

 covered many new faint variables. He had previously 

 stated that no known variable had a fainter minimum than 

 the eighteenth magnitude. He has now found four whose 

 minima lie between the eighteenth and the nineteenth 

 magnitude, and one that goes down to the twenty-first 



The Transit Circle at Greenwich has been fitted with a 

 travelling-wire micrometer for observing transits. This 

 will make a minute alteration in Greenwich time, as the 

 " personal equation " of the old method will disappear. 

 It is found that all observers practically agree in their 

 observations on the travelling method. During a transit 

 the observer continually turns two handles, so as to keep 

 the star bisected by the travelling wire. Contacts are made 

 automatically- when the wire passes certain points in the 

 field. 



BOTANY. 



By Professor F. Cavers, D.Sc, F.L.S. 



SEEDS OF W^TER PLANTS.— It has been stated by 

 different observers that the seeds oi some aquatic plants 

 do not germinate without first being dried ; others have 

 found that the passage of such seeds through the alimentary 

 canal of birds greatlv impn ives their power of germinating . 

 while it has been found that acids and alkalies arouse many 

 water-plant seeds from their dormancy. A closer investi- 

 gation of the peculiarities of the seeds of aquatic plants 

 has recently been made by (rocker and Davis Hot. Gazette, 

 Volume LVIII), who have dealt with the water plantain 

 {Alisrna plan/ago), but consider that their results probably 

 apply to water plants generally. They have studied in 

 this plant the structure and mien 'chemistry ol th< 

 coat, the water relations of seed and embryo, the eit 

 acids on the rate of intake of water by the embryo, 

 imbibition and growth elongation of the embryo, elasticity 

 of the seed-coat, and oxygen pressure in relation to the 

 germination of the seed and the formation of chlorophyll 

 in the seedling. They point out that delayed germination 

 (longevity) of seeds is secured in different way- in different 

 cases : by almost complete exclusion of water in the hard- 

 seeded Leguminosae and other seeds capable of remaining 

 dormant for very long periods: by limiting the degree of 

 swelling ol the embryo by surrounding structure. 

 Alisma, and probably other water plants; by reduction ol 

 oxygen supply below the minimum meessary (or germination, 

 as in various Compositae, some grasses, and soon, and perhaps 

 by deficiency of salt-. To these must be added delays due to 

 embryo characters, as slowness ol acid development in the 

 embryo of hawthorn, apple, and othei Rosaceae. All these 

 possibilities should be considered in Studying the mechanics 

 of delayed germination, instead of assuming, as some writers 

 have done, that injury to the seed-coat or the action of light 

 must act as a stimulus, and that dormancy i- therefore 

 determined bv the characters ot the embryo alone. In the 

 case of Alisma the authors conclude that dormancy i- due 

 to the mechanical restraint ol the seed-COEt, which enables 

 the seed to lie m water tor years without germination V 

 the intact achene in- m water m tin' saturated -tate the 

 embryo itself does not consummate more than one-fourth 

 of its possible iinbibition.il and osmotic -welling and in the 



