May 15, 1884] 



NA TURE 



55 



much resemblance to the vitreous ashes uf Krakatoa, drawn on 

 p. 587, as they are very thin. 



I have examined this sediment with the naked eye to see 

 whether I could perceive anything like the large corona. I 

 darkened the room and admitted the sunlight through a narrow 

 slit on to the glass. The sediment sparkles with various colours, 

 chiefly pink and green, I suppose owing to interference ; and it 

 is difficult to judge which colour preponderates. I find a dei ided 

 excess of green at a small angular distance from the sun, and 

 often pink preponderates ai a greater but varying distance. 

 These colours being similar to those seen in the large corona are 

 slightly confirmatory of the theory that the sediment from the 

 rain is the substance which has caused it and the strange sunsets 

 and sunrises ; but other substances are also capable of giving a 

 green light near the sun. Moisture on glass gives quite different 

 colours, so far as I have observed. 



The cirrus-like wisps on which the sunset phenomena appeared 

 were definite and very small at the end of November ; but on the 

 whole grew larger and more indefinite, till at length they have 

 been quite imperceptible for several weeks past. 



On April 24 there was the first moderately bright aurora I 

 have seen since October 5. Can this remarkable absence of 

 auroras and the scarcely less remarkable frequency of lightning 

 have been caused by the volcanic dust? If so, it may also 

 account for S. Tromholt's finding auroras so scarce and poor in 

 Iceland during the winter, as mentioned on p. 537 (vol. xxix.), 

 though he does not say whether they were scarcer than usual 

 there. Thos. Wm. Backhouse 



Sunderland, May 10 



Pons' Comet 



Pons' comet was visible here with the naked eye throughout 

 the month of February, including the nights of greatest moon- 

 light. I so saw it on some twenty or more nights during that 

 month, and append some notes as to its comparative brightness, 

 so far as I could judge. 



February 3. — "Comet visible till 10.45. Could see 'old 

 moon' with naked eye easily, and in telescope Grimaldi and 

 Aristarchus, but only with a very small part of sunlighted portion 

 in field." 



February 6. — "At 8.45 could see comet with naked eye, 

 though sky not quite free of sunset-glow and somewhat hazy, 

 and moon nine days old. It was altogether faint, but most of 

 the tail visible at other times could be seen — certainly more than 

 I should have expected." 



February 9. — "At 9.30 found the comet with naked eye and 

 could see it without difficulty, but there was only the suggestion 

 of a tail. Comparing it with a Sculptoris by looking midway 

 between the two, they produced the same effect on the eye ; but 

 of course the least magnifying power showed the difference." 



February 10. — " 8.20 to 8.50. Found comet with naked eye, 

 but it was very faint, and to the unaided eye looked certainly 

 fainter than a Sculptoris. Yet it seemed to me that more of the 

 tail (or the tail more certainly) was visible than last night." 



February II. — "Found comet with naked eye about S.to, 

 and watched it up to 9.40. As the sky lost the traces of sunset 

 I could pick it up without difficulty, in spite of the full moon 

 shining in a cloudless sky. h was not quite so easily seen as 

 a Sculptoris, but I may say that A. 1 and A- Sculptoris, though each 

 marked as of the same magnitude as o, I could not get a glimpse 

 of, though I tried hard." 



February 12. — "At 8.15 found comet with naked eye without 

 difficulty, and so at intervals up to 9. Found it again with dif- 

 ficulty at 10.15 ■ '' was 'hen getting low and into the haze : in 

 the telescope it seemed then to have lost (at a guess) half its 

 light." 



On the subsequent clear nights in February there was no 

 difficulty. 



March has been much cloudier, and owing to this and moon- 

 light I only saw it with naked eye certainly on four nights — the 

 1st, 4th, 16th, and 1 7th. My note for the 4th is : "Found and saw 

 comet with naked eye several times, though not easily, between 

 8.15 and 8.40 p.m. Could see the outline of the 'old moon' 

 without difficulty." 



The 5th is marked as doubtful both as to comet and "old 

 moon." 



March 14. — "A fine pink glow in evening, and splendid after- 

 glow about 7.15 — never saw it better. Found comet easily with 

 opera-glass, but could not see it with naked eye, the moon rising 

 before the glow had vanished." So also on the 15th. 



March 16. — "Saw cornel repeatedly with naked eye (looking 

 a little above it) between 7.45 and 8.30. Sky very good." 



.March 17, 7.40 to 8.IO. — " Found comet with naked eye, and 

 saw it many times, looking a little above it ; could not be quite 

 sure of seeing it direct. 



Owing to clouds I have only seen it on two nights since, the 

 24th and 28th ; and that only with opera-glass and telescope. 



Nelson, N.Z., March 29 A. S. Atkinson 



Snow and Ice Flora 



In the account of Prof. Veit Brecher YYitlrock's interesting 

 work on the Arctic snow and ice flora (Nature, vol. xxviii. 

 p. 304) your reviewer enumerates the countries and mountain 

 ranges where red snow has been observed, but does not mention 

 the Southern Alps of New Zealand, where as far back as 1861 

 this plant was observed by me. The fact that green and red ice 

 have been found in these high northern latitudes, and that the 

 unusual coloration has been traced to microscopic organic life is 

 of special interest to me, as 1 repeatedly observed green as well 

 as red ice amongst the glaciers of New Zealand, first at the head 

 of the Rangitata River, as far back as February, 1S61. At the 

 time I published an account of this occurrence, which was re- 

 printed by others (amongst others see Hochstetter's "Neu 

 Seeland," 1S63, p. 342). Since then during my alpine explora- 

 tions I have repeatedly observed the same phenomenon, so that 

 evidently at the Antipodes there occurs a counterpart of the 

 Arctic snow and ice flora. It is to be hoped that some able 

 botanist will some day do the same work for us that Baron 

 Nordenskjold and his able coadjutors have done for Greenland 

 and Spitzbergen. Julius von Haast 



Christchurch, X.Z., December 31, 18S3 



The Rotation Period of Mars 



NOTWITHSTANDING his comparatively small diameter and 

 slow axial motion, the planet Mats affords special facilities for the 

 exact determination of the rotation period. Indeed no other 

 planet appears to be so favourably circumstanced in this respect, 

 for the chief markings on Mars have been perceptible with the 

 same definiteness of outline and characteristics of form through 

 many succeeding generations, whereas the features such as we 

 discern on the other planets are either temporary atmospheric 

 phenomena or rendered so indistinct by unfavourable conditions 

 as to defy lengthened observation. Moreover it may be taken 

 for granted that the features of Mars ate permanent objects on 

 the actual surface of the planet, whereas the markings displayed 

 by our telescopes on some of the other planetary members of our 

 system are mere effects of atmospheric changes which, though 

 visible for several years and showing well-defined periods of 

 rotation, cannot be accepted as affording the true periods. The 

 behaviour of the red spot on Jupiter may closely intimate the 

 actual motion of the sphere of that planet, but markings of such 

 variable unstable character can hardly exhibit an exact conformity 

 of motion with the surface upon which they are seen to be pro- 

 jected. With respect to Mars the case is entirely different. No 

 substantial changes in the most conspicuous features have been 

 detected since they were first confronted with telescopic power, 

 and we do not anticipate that in future ages there will be any 

 il difference in their general configurations. The same 

 markings which were indistinctly revealed to the eyes of Fontana 

 and Iluyghens in 1636 and 1659, will continue to be displayed 

 to the astronomers of succeeding generations, though with greats 1 

 fulness and perspicuity owing to improved means. True there 

 may possibly be variations in progress as regards some of the 

 minor features, for it has been suggested that the visibility of 

 certain spots have varied in a manner which cannot be satis- 

 factorily accounted for on ordinary grounds. These may possibly 

 be due to atmospheric effects on the planet itself, but in many 

 cases the alleged variations have doubtless been more imaginary 

 than real. Tire changes in our own climate are so rapid and 

 striking, and occasion such abnormal appearances in celestial 

 objects that we are frequently led to infer actual changes where 

 none have taken place ; in fact, obsei • 1 too careful to 



consider the origin of such differences and to look nearer home 

 for some of the discordances which may have become apparent 

 in their results. 



The rotation period of Mars has been already given with so 

 much precision that it may seem superfluous to rediscuss the 

 point, but it is very advisable to see whether recent observations 



