June 23, 1910 



NATURE 



487 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.] 



The Tail of Halley's Comet on May 18- 



19. 



Perhaps the following observations I made of Halley's 

 comet on the night of May 18, when it crossed the sun's 

 disc, may be of interest as a record. 



On that evening I crossed by steamer from Palermo to 

 Naples, as I wished to have a clear horizon all round to 

 .see what would happen. The vessel leaves Palermo at 

 7 p.m. and arrives at Naples at 7 the following morning, 

 and it seemed the best place for a view. 



I may say that I had been watching the comet every 

 night from ^Iay 7, and was quite familiar with its appear- 

 ance. I say this because it was quite different from that 

 of any other comet I can remember. The first time 

 I saw the tail was when I came on deck on May 7 about 

 4 a.m. ; the nucleus was not visible, but right across the 

 sky was a long white streak just like a cloud, quite as 

 opaque as a cloud, and I could not believe it was the tail 

 of the comet at all ; but on coming on deck the next morn- 

 ing at 2.15 I saw the same white streak, but this time 

 with the nucleus, also of a very white colour. 



I had no instrument with me to measure the length of 

 the tail, but I got the quartermaster to lay it off on the 

 ship's compass ; he put it down on paper ; it was E. 5 N., 

 and the end of the tail was E. by S. ^ S., which Is about 

 22^° horizontal measurement ; the real length of the tail 

 Itself I could only estimate as " about half-way across the 

 sky." 



On the night of May 18, as soon as it got sufficiently 

 dark, the tail was plainly visible; there was a ten-days' 

 oW moon which rather interfered with t'r.e view, but about 

 ■2 a.m. it had got sufficiently low and behind a thin but 

 convenient bank of cloud, so that it did no further harm to 

 my observation. Of course there was no nucleus to be 

 seen ; that was down below with the sun, but the tail was 

 quite different in character from that which I had seen on 

 the previous nights. It was not a long streak of white, 

 but a confused mass of pinkish light extending along the 

 Tiorizon for 40° or 50°, and then stretching right across 

 f'^° sky, coming gradually to a point at the wide naked- 

 double star (a and 3 Capricorni) below- .'Mtair, in line 



h the three stars. The tail narrowed in on its course 

 upwards, and passed just below the Great Square of 

 r Rasus, y Pegasi being well in the tail, but a Pegasi 

 clear of it. 



: continued watching the tail for shooting stars in its 

 ..-i^hbourhood, but I only saw three or four; there was 

 nothing particular about them, except that thev seemed 

 to start from the edge of the tail, which was well defined, 

 and only travelled 4° or 5° from it. 



But there still remains a curious sight to describe which 

 1 saw on the other side of the ship. 



-About 2.15 a.m. I went aft to get the time from the 



•^^nrt-room clock, and, happening to look over the port 



of the ship to the west, I saw a pillar of light on the 



: losite side of the earth to that from which the comet's 



iail came up; it was about 45° (roughly) high and 50° 



or 60° broad at the horizon ; it was straight up and down. 



1 was much brighter in the middle than at the sides, and 

 bright part seemed like a pillar of light, but the lighter 

 " .d more transparent sides came up and formed a large 

 cone. The setting moon was a good deal to the right of 

 the cone, and was somewhat clouded out. and had no 

 connection with it. .\t the time I took the cone to be 

 the Gegenshein, and did not pay much more attention to 

 >t, beyond looking now and then to see that it was still 

 there. 



Both the cone and the tail were visible from 2.15 to 

 a.m. It is quite possible that at this time the earth 



V have been passing through some of the tail, and had 

 divided it «in two. 



I was up at Monte Casino the next night; unfortunately 

 a fog came down on the mountain, but I heard that 

 at the observatory they had seen an arch of light over the 



part of the horizon from which the tail came. I did not 

 see this ; but I was at sea-level, and the observatory is up 

 some 1500 feet. How.4Rd Pays. 



20 Hyde Park Place, London, W., June 17. 



An observing party was organised at this college for the 

 purpose of taking note of any physical disturbances which 

 might occur during the passage of the earth through the 

 comet's tail, particulars of which will be published later. 

 Our object in now writing is to put on record a remark- 

 able appearance which presented itself at about 3.30 on 

 the morning of May 19. 



The comet has been visible here to the unaided eye since 

 -April 12, and up to the morning of May 18 the tail pre- 

 sented what may be termed a normal appearance, i.e. 

 smaller at the nucleus than at the extremity, but on the 

 morning of May 19 the character had altogether changed. 

 .\t about 3.30 a luminous patch was seen at an altitude 

 of about 20° from the horizon, and in the place where the 

 tail formerly appeared. There were some clouds near the 

 horizon, and as these cleared away the whole of the tail 

 became visible, extending at 4.30 right up to the zenith, 

 and there being lost in the Milky Way. 



When there were no clouds the sky was remarkably 

 clear, the Milky Way shining most brilliantly. The licfht 

 from the tail of the comet was polarised, but not so dis- 

 tinctly as was the case with the normal tail on previous 

 mornings. The tail persisted until daylight. It, to some 

 extent as regards shape, simulated the Zodiacal Light, but 

 at the same time was essentially different, and did not 

 appear in the usual situation of the light, as it was many 

 degrees to the north of the sun. It was much longer, 

 narrower at the base, and ten times brighter. There is 

 no question but that it was the comet's tail. 



.At 4.30 the upper half of the tail was quite free from 

 cloud, and the gradual narrowing towards the upper end 

 was most marked. It seemed from the curvature of the 

 edges that a portion was missing from the under side of 

 the tail. The conviction was borne in upon us that we 

 saw a portion of the tail blotted out or cut off in some 

 way, and this was certainly not done by cloud. Was it 

 done by the earth's atmosphere? The following morning 

 was cloudy, and nothing was seen at 3.30, but the comet 

 appeared in the western sky at 5.40 in the evening. 



Observations were not taken on succeeding mornings, 

 which perhaps was a mistake, as something may have 

 been 'eft behind after contact with the earth, if contact 

 really happened. 



W. H. F1XL.0. 

 W. -A. Douglas Rudge. 

 University College, Bloemfontein. 



NO. 2I2T, VOL. S^l 



The Colour of Pure Water. 



Having noticed the colour of the sky, of air, and of 

 water under different conditions, I was reminded on read- 

 ing the report of Lord Rayleigh's lecture (Nature, 

 March 10) of a few notes I had made from time to time, 

 and now think they may prove of interest. 



First, optically pure water cannot be obtained by dis- 

 tillation. Prof. Tyndall asked me to prepare some pure 

 water for him, which I attempted, first by distillation 

 with acid permanganate, and then re-distilling this from 

 a copper vessel and collecting the liquid in a bottle placed 

 in a large bell-iar of hydrogen, a gas which is known to 

 provide an optically pure atmosphere. The resulting water 

 was not optically pure. Pure water was prepared bv 

 Tyndall by melting clear block-ice in a vacuum. Its colour 

 was blue when seen through a tube 3 feet long. 



The colour of a hard water which has been softened bv 

 Clark's process may be seen at the Colne Vallev water- 

 works, visible from the train on the up line just south of 

 Watford Station, and at Joynson's paper works at St. 

 Mary Cray, in Kent. When the members of the Society 

 of Chemjcal Industry visited these works some vears ago, 

 they were much struck by the very beautiful blue of the 

 water. It was even suggested that it had been purposely 

 coloured with a very pure blue dye. Water of similar 

 purity, containing very little mineral matter, being re- 

 maikable for its softness, comes from the Greensand 

 h-low the London Clay. Such blue water I have noticed 



