Sept. ii, 1884] 



NATURE 



463 



found a solar dust-halo, with effects like the above, more and 

 more visible the higher he ascended amongst the Alps. 



That such appearances were produced by solid particles in a 

 cold state, and not by any new gas introduced into the atmo- 

 sphere, seems to be borne out by three sets of rather extensive solar 

 spectroscopings which I have lately carried out ; for while there 

 does not seem to be a single new line amongst the thousands of 

 old ones, so far as I have yet examined the observations, there is 

 only too abundant evidence of a continued dulling of the light 

 of the sun's continuous spectrum all along its range. 



This effect is of course more conspicuous in the faint regions 

 at either end than in the bright middle, and would appear to be 

 testified to undeniably by the following differential observation, 

 viz. that with a prismatic apparatus, wherewith I could see lines 

 in the bright regions, say of B, C, and D, rather better than I 

 could with somewhat similar, but darker, prisms in 1877, — I 

 could not see Brewster's line Y and its companion groups in tire 

 very faint ultra-red so well as I did then ; and could not see the 

 further-away line X at all, though in 1877 it was not only clear 

 enough, but far fainter lines on either side of it were visible and 

 micrometrically measurable. Neither in 1884 have I been able 

 with the same eye and instrument to see anything at the violet 

 end of the spectrum of the grand banded lines H and K, though 

 they formed a daily subject of observation in 1S77. 



In 1S56 I remarkably appreciated that an ascent to 11,000 

 feet of altitude on the Peak of Teneriffe enabled II and K to 

 be seen with peculiar distinctness and fine resolution of much of 

 their haze at lower levels into sharp lines ; but would that 

 have been equally the case this year, when the inhabited regions 

 of the earth, and the lower clouds too, are covered in by a wide- 

 spread blanket of dust in most anomalous extent and density ? 

 C. Piazzi Smyth 

 Astronomer- Royal for Scotland 



15, Royal Terrace, Edinburgh, September 6 



Pons' Comet — Pink Glow 



This comet was visible here up to the beginning of June. I 

 saw it on fourteen nights in April and eighteen in May, including 

 the last eleven nights of the latter month. It could be seen with 

 an opera-glass up to April 3 ; my last sight of it was with a 

 4-inch telescope on June I, or rather at 12.30 a.m. of June 2 

 (= June id. ih. G.M.T.). On April 24, and again and par- 

 ticularly on May 24 it seemed to me to have become suddenly 

 fainter, though there seemed nothing in the state of the sky to 

 account for it ; indeed, on the last-named night I have noted, 

 " sky very clear." Up to at least May 28 its motion in two or 

 three hours could be plainly seen. On that night, though "very 

 diffused and faint," it was visible before the moon had set. It 

 had not, I think, on June 2 reached the minimum visibile, but 

 as I had no ephemeris subsequent to that (to the middle of 

 April) given in Nature, it would have been quite useless to 

 have looked for it again after the moon had passed. 



I may add that the "pink glows" have not yet left us; on 

 the last two evenings (July I and 2), which were clear, they 

 were very distinct. A. S. Atkinson 



Nelson, N.Z., July 3 



Alternation of Generations in Salpa 



While we are indebted to Prof. W. K. Brooks for having 

 enunciated his views on this subject clearly in Nature for 

 August 14 (p. 367), I should like to point out that the mis- 

 quotations which he has called attention to in an article of mine 

 published in May (p. 67) do not invalidate the strength of the 

 counter-arguments, although I must apologise for their having 

 been allowed to appear. 



He does not acknowledge that the question at issue is one not 

 of fact but of the explanation of accepted fact, i.e. it is a ques- 

 tion of theory. Undoubtedly an egg migrates from the body 

 of the solitary Salpa to that of the chain form, but Kowalevsky, 

 who himself describes this, does not agree with Prof. Brooks' 

 conclusion drawn therefrom. 



Prof. Brooks pointed out at greater length than I did that the 

 phenomena found in Pyrosoma and Composite Ascidians cul- 

 minate in those in Salpa. Beginning in Pyrosoma with "an 

 indefinite series of hermaphrodite buds," he shows how the re- 

 productive cell becomes marked out earlier and earlier, until in 

 Salpa it is fully developed in the body of the gemmating 

 individual. Then, instead of showing by his nomenclature that 



Salpa is the end of a series, lie prefers to break loose from 

 any attempt at continuity and to call the solitary Salpa a true 

 female. 



I, however, prefer to follow in the steps of Prof. Moseley, 

 who says of similar changes in the Hydromedusse, that " it 

 would lead to great confusion if the old way of regarding the 

 matter was upset. The past history of the gonophores must be 

 taken into account, and the fact that the sexual elements, though 

 now developed at a greater or less distance in many species, 

 formerly undoubtedly originated within the gonophore." 



As Prof. Brooks does not use language in this way, it is not 

 remarkable that he criticises me for using the term " hydroid " 

 in regard to Cunina at a stage comparable to the hydriform and 

 gemmating person of a Sertularian, although I pointed out that 

 it is a Medusa in both generations. 



The fault of Prof. Brooks' argument is that he is not consistent. 

 He says : " Very many chain Salpsa are produced at one time. 

 As these have no power to reproduce by budding, they have lost 

 t/h ir ovaries, although each of them when it is born contains, 

 like the bud of Pyrosoma, a single unfertilised egg." 



If this means that the egg is the sole remnant of the ovaiy, 

 it admits all that I contend for ; but if, on the other hand, it 

 means that in a less modified condition these must have an 

 ovaiy proper to the bud as well as the ovum received from the 

 solitary Salpa, it follows that Salpa cannot be differentiated 

 from a form like Pyrosoma, where there is, so to speak, a 

 migrating ovary, but no trace of ovary independently formed in 

 the bud. The second ovary described by Salensky cannot be a 

 trace of this, for it is simply another ovum with follicular cover- 

 ing precisely like the first. R. N. Goodman 



St. John's College, Cambridge 



Forked Lightning 



By papers received by last mail I see that Mr. W. C. Gurley 

 claims to have shown, by photographing a flash of lighthing, 

 that the ordinary notion of forked lightning must be given up. I 

 do not know whether this conclusion has been drawn from the 

 photograph of a single flash or not, but you will see from the 

 inclosed photographs that the conclusion is an entirely false one. 

 An examination of my photographs will show that all the flashes 

 except one had the zigzag form, and that some of them are mag- 

 nificently forked. They resemble very closely the photographs 

 of sparks from a Holtz electrical machine, taken by Mr. A. 

 Matheson in Prof. Tait's laboratory, and published in vol. xxvii. 

 Part 3, of the Transactions of the Royal Society of Edinburgh. 

 The amount of detail shown in the photograph of the tree 

 illuminated by the flash gives one a very good idea of the bright- 

 ness, when we consider that exposure cannot have exceeded the 

 millionth part of a second. I may add that my first photograph 

 was taken on October 16, 1883, and was circulated amongst 

 friend- immediately afterwards. C. MlCHIE Smith 



Madras Christian College, Madras, August 9 



Sun-Glows 



As one of the first to draw attention in the St. James's Gazette 

 of October 1, and November 9, 1883, and many subsequent oc- 

 casions, to those strange phenomena about the sun last autumn, 

 will you kindly allow me space in your valuable columns to ask 

 how it is possible to refer such effects any longer (as Mr. Back- 

 house does in your paper of August 14, p. 359) to volcanic dust 

 from, I presume, the Krakatoa eruption, when we know now 

 that in south latitudes these phenomena were observed by Mr. 

 Neison of the Natal Observatory as early as the spring of 1883? 

 He says that " they increased in intensity from February until 

 June, when they were strongly marked." I have watched the 

 sky as an artist (out of London) for quite forty years, and feel 

 sure that this corona, or blanching of the sun, has been a more 

 persistent feature of late years than formerly. It is still there, 

 and may be seen without leaving England, or even London in 

 clear weather, by looking for it from about an hour to half an 

 hour before or after sunset and sunrise. The last very mild 

 winter and the preceding one could have had no connection 

 with the Krakatoa eruption, and I think that we must now seek 

 for an explanation of the present and past atmospheric pheno- 

 mena in some increase of solar energy, and consequent lilting of 

 vapour higher than usual. Robert Leslie 



6, Moira Place, Southampton, August 24 



