February 1, 1897.] 



KNOWLEDGE. 



53 



opportunities of carefully examining the phenomenon. 

 When the zodiacal light had been most intense, I have 

 observed that it would be perceptibly weakened for a few 

 minutes, until it again suddenly shone forth in full 

 brilliancy. In some few instances I have thought that I 

 could perceive, not exactly a reddish coloration, nor the 

 lower portion darkened in an arc-like form, nor even a 

 scintillation — as Mairan afiirms he has observed — but a 

 kind of flickering and wavering of the light." These 

 rapid apparent changes, however, he thinks, may be partly 

 due to causes acting in the upper strata of our atmosphere. 



Olbers convinced himself " that this light is very 

 different in diii'erent years, often for several successive 

 years being very bright and diffused, whilst in other 

 years it is scarcely perceptible." This, of course, is a 

 different sort of variation from the temporary or momen- 

 tary changes just mentioned. Some modern observers 

 think it decidedly variable in the way mentioned by 

 Olbers — that is, more brilliant on some occasions than on 

 others. But we must bear in mind two things in con- 

 sidering this question. First : as seen in England the 

 total apparent length of the cone decreases gradually 

 as the season of visibility goes on. Thus, for the evening 

 apparition the length as observed in March and April is 

 nearly always markedly less than when seen in January 

 and February.* Hence, because the length of the cone 

 appears different as seen at dates separated by an interval 

 of (say) a fortnight, it is not necessarily to be inferred 

 that the briiliancy has changed in that interval of time. 

 Second : we all know how after a spell of dull or wet 

 weather an unexpectedly clear night suddenly turns up. 

 On the last occasion of a clear but " poor '' night, the 

 zodiacal light perhaps looked poor too. Now, on the 

 renj clear evening, it is seen under fine conditions, and 

 naturally the finer and more delicate parts stand out 

 clearly. But this is not variability. We cannot focus our 

 sight on to this delicate object as on a variable star, and 

 the circumstances are such as do not admit of the applica- 

 tion of exact photometry to the case. To decide this 

 question we need a regular series of systematic observations 

 made in some clear and serene sky, like that of Peru or the 

 Transvaal, night after night and year after year. Such 

 observations should give, according to a fixed method, the 

 length and breadth ; also the brightness, say at certain 

 fixed altitudes, as compared with the Milky ^^'ay. It is 

 only from such a record that the question of variability can 

 be settled. Very possibly, changes in the density of 

 different parts of the zodiacal meteors or cosmic dust may 

 take place from time to time, and consequently changes in 

 brightness. But I do not think that so far they have been 

 fairly proven. 



Some observers have noted a ruddy tint, but I confess 

 that I have never detected this. 



Good observers, like Admiral Smyth, acknowledge many 

 disappointments of not catching it ; and to anyone who has 

 not yet seen the light, but wishes to do so, I would offer 

 the following suggestions : — Time, middle of -January to 

 middle of March ; locality, remote from any large town or 

 collections of lights. From some of the breezy commons 

 and downs of Surrey or Sussex, well to the south of the 

 metropolis and free from trees, one can command a 

 grand sweep of sky. Better still if the observer has a 

 sea horizon to the west, for then, provided there is no 

 shipping, he may be absolutely certain that no artificial 

 glare exists. Choose a brilliant evening such as some- 

 times comes after heavy rain. There must be no trace 



* See observations of Prof. Pereira and self in Journal Slit. Asl. 

 4ssoc., Vol. v., pp. 361, 42X. 



whatever of moonlight, which blots out our faint subject. 

 Keep on the look-out, say, from a quarter of an hour after 

 sunset. Do not strain the sight by looking fixedly at a 

 star, or in one place. Cast the eye occasionally over the 

 western sky towards the horizon, and with the above cir- 

 cumstances favourable you should have no difficulty in 

 " catching " the phenomenon and " holding " it, too. I 

 am not quite prepared to say whether near-sighted persons 

 would be handicapped in this search. Possibly spectacles 

 may in some way impede vision in sweeping over a large 

 area of the sky. There must be a slight loss of light, even 

 through one lens only. Personally, I am blessed at 

 present with a long sight, which does not need any assis- 

 tance in the way of glasses. When once seen, it is 

 comparatively easy to see it again another night, as we 

 then know what sort of a thing it is we are looking for. 



It is curious that the zodiacal light has not been seen 

 during a total eclipse of the sun, and it would therefore 

 seem that its light is decidedly inferior to that of the 

 corona. At the same time, in such a consideration one 

 must not omit to take account of the vast area of 

 illuminated earth and sky which surrounds the moon's 

 shadow cone. The darkness of an eclipse is not that of 

 night. 



Would an observer from Neptune, supposing he could 

 do without any atmosphere himself, see the sun as a 

 " nebulous star," with the zodiacal light extending on 

 either side Uke a delicate comet? Very probably — with 

 many a comet ploughing through it, which our own 

 atmospheric veil prevents us by its brilliant glare from 

 seeing. Take as an instance the comet seen close against 

 the sun during the total solar eclipse visible in ligypt 

 in 1882. 



There are two other phenomena connected with the 

 zodiacal light, viz., the r/ivienschein, or counter glow, and 

 the zodiacal band. The first is a faint patch of light seen 

 very nearly opposite the sun's place ; the other is a 

 prolongation of the zodiacal light to the gei/enscheiu. 

 Both these are excessively delicate phenomena and require 

 the finest sight and sky. Although I consider I have a 

 good eye, I have never seen either. Even Prof. Pickering 

 can only say that " the evidence for the actual existence 

 of some light opposite the sun becomes tolerably strong." 

 Still, there can be but little doubt of the existence of 

 the :/f</tnsrhein from the observations of Brorsen, Schmidt, 

 Heis, Backhouse, Lewis, Barnard, and others. But the 

 ordinary observer can hardly hope to detect this or the 

 zodiacal band. 



EVIDENCE OF THE EVOLUTION OF STELLAR 

 SYSTEMS. 



By Is.v.^c Roberts, D.Sc, F.R.S. 



THE answers which to me appear to be relevant to 

 the questions propounded in the first part of this 

 communication, will be found by the correlation 

 of the annexed photographs of four spiral nebute 

 with the four photographs of stars which were 

 given in the last number of Knowledge. 



No. 1 is of the spiral nebula in Pisces, R.A. Ih. .Sim., 

 Decl. 15^ 15' north. Exposure, 8h. 40m. on the 9th 

 December, 1893. Scale, one millimetre equals twelve 

 seconds of arc. 



No. 2, spiral nebula in Ursa Major, R.A. 13h. 59m., 

 Decl. 55'^ 10' north. Exposure, 3h. "iOm. on the BOih 

 May, 1892, Scale, one millimetre equals twelve seconds 

 of arc. 



