Jan. i, 1885] 



NA TURE 



'93 



on them when they were very far from the sun, they then being 

 simply white. I did not see any dark ones, as described by J. 

 E. Clark ; indeed they always struck me as being very thin, 

 merely like a nearly flat sheet. They tended to be arranged in 

 bands like "Noah's Arks," and, while their texture was 

 smoother than most cirrus clouds, they were more or less striated 

 transversely. On some afternoons 1 noticed in many cases a 

 feeble smoke-like prolongation, or tail, 071 the east side of the 

 cloud ; this had no colouring. They had thus sometimes a 

 striking resemblance to an aurora, differing essentially, how- 

 ever, in their real position being horizontal, while the auroral 

 band and rays are almost vertical. Their direction also was 

 quite different : on the nth at S'I5 a.m., and 13th at 3 .40 p.m. 

 I noticed that the stria; pointed to east by south. In shape they 

 approached parallelograms apparently ; really, to rectangles ; 

 sometimes they were very perfect rectangles 1 >ne of the most 

 striking clouds was, however, a perfect right-angled triangle in 

 form. Their motion was very slow. Some time after sunset 

 they were so bright as to give a material amount of light, and 

 to make the dust-circle around the sun look quite dim. They 

 were evidently at a great height, though they looked lower than 

 the dust-wisps. They were incapable of producing an ordinary 

 halo. 



Like Prof. C. Piazzi-Smyth, I can say that I have no recol- 

 lection of seeing any clouds of the kind before. I saw nothing 

 like them at the time of the grand sunsets last autumn, and I 

 think he is mistaken in supposing any of the phenomena then 

 seen were of the same character. T. W. Back I 



Sunderland, December 22, 1S84 



Referring to the letters which have appeared in these 

 columns on the subject of "Iridescent Clouds" as seen at 

 Edinburgh and York on the evening of December II, a very 

 similar phenomenon was seen at Derby at sunrise on that day, 

 and was thus described in the Derby Express the same evening : — 

 "About half an hour before sunrise the eastern half of the sky 

 was covered with a dense pallium of cirrus cloud. About 30° 

 above the horizon was seen what appeared to be an elongated 

 opening in the dark grey of the cloud. Through this spindle- 

 shaped opening the sky was of an intense erne: aid colour. The 

 strangest part of the phenomenon, however, occurred shortly 

 before eight o'clock, when the vivid green had given place to a 

 mass of brightness comprising all the prismatic colours arranged 

 in bands transversely, each of the primary colours shading 

 gradually into its neighbour in the same manner as in a solar 

 rainbow. The appearance was now not unlike a huge many- 

 coloured eye set in a dark uniformity of cirro-stratus. As the 

 sun arose the colouring faded, and when the solar orb was several 

 degrees above the horizon the phenomenon remained as a patch 

 of brightness upon a silver-grey vapour, and was somewhat 

 similar in appearance to an imperfectly formed parhelion. Its 

 in, however, with regard to the true sun, showed at once 

 that the phenomenon was not of the parhelion class " 



C. J. P. 



The iridescent cloud effect mentioned by your correspondents 

 (see Nature, p. 148) was well seen here on the 13th about 

 4 p.m., and was very much as described by Mr. Clark. Three 

 distinct bands of colour were seen just at the upper edge of a 

 dark slate-coloured cloud towards south-west, and two faint ones 

 on the clearer sky above. I write specially to remark on the 

 nature of the colour of these bands. They were not prismatic 

 colours as mentioned by Mr. Clark, but unmistakable inter- 

 ference or residual colours, the lowest bright purplish pink, 

 shading into green, the next the peculiar light brick red seen in 

 Newton's rings, and a very recognisable colour, also shading into 

 green, and the rest pink and green, of similar colour to the 

 lowest. There can be, I thiuk, no question that this was an 

 interference-phenomenon, and 1 hope some of your correspond- 

 ents may be able to give the rationale of it. 



Fairfield House, Darlington James I'anson 



I see notes in Nature, December 18, 18S4 (p. 148), on 

 iridescent clouds. I observed similar appearances on the York- 

 shire Wolds, between Market Weighton and Brough, on 

 December 6 and again on December 13, 3-4 p.m. ; but instead 

 of the clouds being totally coloured, only the edges of rifts in a 

 thick cloud-mass were so tinged. The phenomenon was much 

 finer on the latter date, the rift being much larger and the 



colours more widely dispersed at one end, so that a rose tinge 

 occupied there the whole of the acute angle of the gap. 



Broseley, .Shropshire \V. W. Watts 



The Rotation of Neptune 



Several circumstances delayed my observation of the planet 

 Neptune this autumn until November 24. On that and the two 

 following nights the light of Neptune was compared with the 

 light of the star B.A.C. 1072 ; and, assuming that the light of the 

 star was steady, that of Neptune was found to undergo appa- 

 rently regular vaiiations, but much smaller than they were last 

 year. 



The observations were combined in the following manner : — 



The magnitude, ///, at any time, /, was assumed equal to 



m -' '• sin n {/ - /J, 



where 111. was the mean magnitude at the time /„, k one-half the 



variation between maximum and minimum, and n equal to 



- — , or 4S°'45, according to the observations of Neptune last 

 7-92 



year, which gave 7-92 h. as the rotation-period. Subtracting ml, 

 the unknown magnitude of the comparison star, which is, how- 

 ever, of about the seventh magnitude, we have 



111 - in' = m - in' + k sin 11 (t - /„) ; 



and by assuming approximate values, by introducing corrections, 

 and by solving the n equations corresponding to the n observa- 

 tions by the method of least squares, it was found that 

 m - m' = 0'86 



/• = CTI9 



/„ = Nov. 24.I. 13-011-1. G.M.T. 

 The preceding epoch of maximum will lie found by subtracting 

 i"98h. ; and similarly the following epoch of minimum will be 

 found by adding I -gSh, 



Now these observations were made without special care, and 

 consequently the probable errors were larger than they should 

 be in comparison with the small variation ; but on the night of 

 November 29 every care was taken to obtain accuracy in the 

 photometric measures, and the following results were ob- 

 tained : — 



m_ - in' = o 82 



k — 0'20 



t = Nov. 29d. 1172I1. G.M.T. 

 The following is the comparison of observation and computa- 

 tion : — 



Diff. mag. 



Kempshot M.T. 



18S4 Nov. 29 

 h. n . 



Obs. 

 0'9I 



1 



7 10 



S 26 ... 1 -03 

 10 5 ... o-8S 

 12 7 ... 064 

 It should perhaps be added that Kempshot is 5'io.h. west of 

 1 Greenwich. 



By comparing the epoch on November 29 with the corre- 

 sponding epoch on November 24, we find that 15 rotation- 

 periods occupy 118-7111., so that each rotation-period is 7-91411., 

 which may be considered identical with the period found last 

 year. Maxwell Hall 



Jamaica, December I, 18S4 



Peculiar Ice-Forms 



CIRCUMSTANCES have prevented my replying earlier to Dr. 

 1 in Nature of November 27 (p. 81). The situa- 

 tion of the ice described in my letter of November 6 (p. 5) 

 precludes the possibility of its having been a remainder from last 

 winter's snow, since it was only some fifteen hundred feet above 

 the valley of Chamounix, and exposed during the summer months 

 to daily sunshine. In fact, the mid-day sun only just failed to 

 reach it on the 17th of October. 



In the Neues Jahrbuch jiir Mineralogie for 1S7 7 (referred to 

 by Dr. Wetterhan of Freiburg in Nature, vol. xxi. p. 396) is 

 an article by Dr. G. A. Koch giving an elaborate description 

 and discussion of a very similar ice-structure, formed under very 

 similar circumstances, which he observed on October iS, 1875, 

 near St. Anton in the Arlberg. He also quotes other cases 

 I rved on the Wormserjoch in the Tyrol, and by Prof. Doenitz 



