432 



KNOWLICDGK. 



NOVEMIIER, 1912. 



oin" ihouHiiml million stars in Ihi' siden-al sysloiii, so that wc 

 rL-<iiiiri' a star density in the outer portion more than half a 

 million times as great as in oiir own neiKhbonrhood, which 

 is certainly an iniprobatilo arranucmont. Moreover, such 

 extreme crowdinR would make llic uuitnal Rravitaliou of the 

 galactic stars appreciable, and would render the absence of 

 rel.itive motion still more unlikely. 



(41 The nebula round Nova I'ersei gives us a fairly reliable 

 estimate of the distance of a portion of the Gala.xy. We have 

 to make two assumptions, both of which seem to me probable : 

 ((») that the nebulosity was rendered visible to us by the 

 reflection of light from the Nova ; ib) that the Nova was 

 really in the Galaxy, not merely between us and it ; this is 

 deducible from the fact that Novae, almost without exception, 

 have appeared in or near the Galaxy. (I think the only 

 exception is Nova Coronae.) It is well known that, even 

 assuming (a), the distance of the Nova is not iunnediately 

 deducible. This distance depends on the angle Earth-Nova- 

 Nebula. Calling this angle <P. and calling a the apparent 

 angle through which the Nebula moved outwards from the Nova 

 in a year, a simple trigonometrical calculation gives us the 



distance of the Nova in light years " ^ 



sm a + sin — sin (a + 0) 

 I take a, from approximate measures of the photographs of 

 the nebula, as 14' -6, and deduce the following values of the 

 distance in light years corresponding to different values of 0. 



Kapteyn, from some very probable reasoning based on the 

 forms of the nebular outlines in the photographs, adopts 79° 

 for iAstro. iXacIi. 3756, and Pop. Astruii.. 1902, March). 

 This implies a distance of two hundred and eighty-tive light 

 years, and a parallax of 0"-0n. It seems highly probable 

 that the distance of Nova Persei is not more than some four 

 himdred light years. The small values of implied by the 

 assumption of a greater distance would imply that the nebula 

 did not surround the Nova, but was several light years on the 

 near side of it, which is an artificial and imlikely configuration. 



But a distance of four hiuidred light years for the nearer 

 parts of the Galaxy is not inconsistent with Newcomb's estimate 

 of three thou.sand light years for its further parts. It may 

 well have several coils, and their depth in the line of sight 

 may considerably exceed their breadth. 



PHOTOGR.APHV OK IHI': MOON IN LLTR.\. 

 VIOLET LIGHT.— Mr. R. \V. Wood has been for many 

 years experimenting on the different results obtained in photo- 

 graphing the moon with light of different wave-lengths. He 

 describes his latest results in Tlic Astrophysical Journal for 

 July. He uses a nickel-on-glass reflector, and obtains the 

 ultra-violet images by covering the plate with a screen of 

 uviol glass, one millimetre thick, coated with silver; this is 

 opaque to all but ultra-violet rays. For comparison he took 

 ordinary short exposure photographs without any screen 

 (these are chiefly formed by violet light! and orange exposures 

 with Cramer Iso plates and a deep orange screen. He tried 

 the effect of making a three-colom- picture from the three 

 negatives, using red, yellow and blue pigments to represent the 

 orange, violet and ultra-violet negatives. He states the result 

 was "a vety pretty colour photograph, which brought out the 

 difference of reflecting power of the ditTerent maria in a very 

 striking manner. The prevailing tone of the darker portions 

 of the lower surface was olive green, but certain spots come out 

 with an orange tone and others with a decided purple colour." 



.\ remarkable spot near Aristarchus was foimd to be 

 invisible in yellow light, faint in violet, very dark in ultra- 

 violet. By experiment he found that volcanic tufi" stained 

 with sulphur gives this effect and suggests that sulphur had 

 been deposited in that region by a volcanic blast. The new 



ruethod seems to aHord a lever by which, in time, considerable 

 itiformation may be gained as to the constitution of the Moon's 

 surf.ice rocks. 



COMETS.— Mr. Walter F. Gale, of New South Wales, the 

 discoverer of Comet 1894 II., found another comet on 

 September 8th last. It has been visible to the naked eye, 

 with a bright nucleus, a considerable coma and a short tail. 

 Perihelion passage I912.0ctober 4 -96, Greenwich M.T.. Omega 

 25" J6'. Node 296" 56', Inclination 79° 54', Perihelion Dis- 

 tance 0-7164. 



Ephemeris for 1 1 p.m. 



The Comet should be looked for in the West as soon after 

 sunset as it is dark enough. It was visible in October in con- 

 siderable twilight, but will be much fainter in November, since 

 it is receding from both Earth and Sun. 



Tuttle's Comet, whose period is thirteen and two-thirds years, 

 comes to Perihelion about January 3rd next, and will be 

 well placed in December. 



BOTANY. 



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



BEES AND GREEN FLOWERS.— As is well known, 

 various flowers are visited by insects despite the absence of 

 white or brightly coloured petals ; in fact, some flowers in 

 which the floral leaves are green, like the ordinary foliage- 

 leaves, are visited by bees for pollen. Plateau and other 

 writers on this topic have gone so far as to assert that all 

 flowers might be as green as their leaves without their pollin- 

 ation being compromi.sed. Lovell (American Naturalist, 

 1912), has made various experiments with bees and flowers, 

 and objects to which honey had been added, and from his 

 results concludes that (II green flowers are not well adapted 

 to insect pollination, many if not all such species having been 

 derived from larger and more highly developed forms by 

 degeneration; (2) any surface, whether bright or dull coloured, 

 on which there is honey will be freely visited by bees after the 

 honey has been discovered ; but it will not be discovered so 

 quickly on a surface that does not contrast in hue with its 

 surroundings as on one which does so contrast ; (3) the ex- 

 periments of Plateau on green or greenish flowers .-ire falla- 

 cious ; (41 when honey-bees are given the choice between a 

 conspicuous and an inconspicuous object under similar con- 

 ditions, they show a marked preference for the former, and 

 this preference is strong enough to .account for the development 

 of colour contrast in flowers. 



LEAF-MOVEMENTS IN OX A LIDACEAE.— The 

 familiar " sleep-movements " of the leaves of Wood Sorrel and 

 other members of the Oxalidace.ae h.ave been investigated by 

 Pfeffcr, Darwin, Sachs, and other writers. An interesting 

 account of new experiments on these movements is given by 

 Ulrich (Trans. Bot.Soc. Pennsylvania. 1911), who concludes 

 that light pl.ays the most important part in the normal move- 

 ments of the leaflets of Oxalidaceae, heat and humidity being 

 secondary ; the leaflets reach their highest points soon after 

 sunrise, and then sink again for a short time, the rest of the 

 movements during the day depending upon the conditions of 

 their environment; in darkness the movements do not cease 

 until the leaflets have become degenerated ; in blue light, 

 leaves usually oscillate less than in white light, but in a week's 

 time they become almost normal ; electric stimulations give 

 the most uniform results in experiments, because they can be 

 so accurately repeated ; some species assume their sleep 

 position by a series of drops followed by partial recoveries ; 

 the rising of the leaflets in the morning is due to the stimula- 

 tion of the light of the preceding day. which by its constant 

 repetition has made these movements hereditarj- — they con- 

 tinue, when once inaugurated, in the absence of such 

 stimulation. 



