June i6, 1910] 



NATURE 



459 



less safe to assume that there is some foundation for 



doing so, and I should be very glad to learn when and 



re Koch made the suggestion that the crocodile is a 



rvolr host for sleeping sickness. The point is merely 



o!v,- of historical and bibliographical interest, since in his 



latest writings Koch expressly repudiated the idea of any 



such connection between crocodiles and sleeping sickness 



(see Nature, February i8, 1909, p. 458, and May 5, 1910, 



p. 279 ; also the Bulletin of the Sleeping Sickness Bureau, 



II, November 5, 1909, p. 421, footnote). 



E. A. MixcHix. 

 Lister Institute of Preventive Medicine, Chelsea 

 Gardens, S.W., June 8. 



ROF. Minchin's letter comes as a reminder, so often 

 I'pcated, and apparently not too often, that one should 

 verify one's references. I had read a leading article in 

 the Lancet of October 30, 1909, on the work of the German 

 Commission, but I had not referred to the original report, 

 and I am afraid that the statement that the Glossina 

 palpalis, on occasion, takes its nutriment from the unpro- 

 tected parts of the crocodile led me somewhat astray as to 

 the exact significance of Koch's observations and recom- 

 mendations. I am glad that Prof. Minchin has directed 

 attention to the matter. My excuse must be that the article 

 had to be in the hands of the editor within a few hours 

 of my receipt of a re-directed teFegram, and that I had 

 to depend upon my memory for almost everything but a 

 few dates and data which appear in " Wer ist's. " 



The Writer of the Article. 



these corpuscles. This being so, it is impossible for the 

 earth to go through the tail of a comet — it simply repels 

 the tail, and, as a consequence, instead of a passage 

 through it, a disruption near the time of passage must 

 occur, one part being left in the (in this case) morning 

 sky, whilst a new one is developed in the evening sky. 

 Here I may remark that on the evening of May 20 the 

 measured length of the new tail was 19°, on May 21 32°, 

 and on May 22 it was 40°. 



Again, the earth is bombarded with meteorites, which 

 are also throwing off corpuscles. These will be repelled 

 by both earth and sun, so that if we look at the part of 

 the sky opposite to the sun we should, and do, see the 

 faint tail thus formed which is known as the Gegen- 

 schein. This simple theory explains all the facts of 

 observation, and, if it is correct, will save nervous in- 

 dividuals some worry when the next near approach of a 

 comet's tail is imminent. R. T. A. IxxES. 



Transvaal Observatory, May 22. 



P.S. — -Mr. H. C. Reeve, of Lorentzville, under date 

 May 22, has sent me a letter conveying the same idea. 

 He says : — " Whatsoever nature the stress between the sun 

 and the comet may be which causes the repulsion of the 

 tail . . . the same stress must also exist between the earth 

 and the comet . . . under these circumstances the earth 

 could not possibly pass through the comet's tail." 



The Eaith ard Comets' Tails. 



I\ spite of the unreserved predictions of astronomers, the 

 earth did not pass through the tail of Halley's comet on 

 May 18-19, noi" subsequently. The tail as seen in the 

 morning sky, previous to the transit of the comet across 

 the sun's disc, appeared like a long and straight beam of 

 light stretching from the horizon to .Aquila. It was noticed 

 from daj' to day that the tail was practically fixed in 

 position in the sky. We rather expected the tail to get 

 nearer to Venus and Saturn as the comet approached the 

 ecliptic, but it remained stationarj'. On the morning of 

 transit. May 18-19, ^^e tail was unchanged, but a second 

 branch to the south was now noticed. It joined the 

 "■"-thern branch to the east of the Square of Pegasus, 

 ortunately, this southern branch was near the zodiacal 

 r, and only distinguished from it with difficulty. 



Both these tails were seen morning by morning, in- 

 -uding this morning (May 22, Civil day), but they have 

 diminished in brightness, and were difficult to see. Further 

 observation of these will be impossible, because of the 

 moon remaining above the horizon until after dawn during 

 the next ten days. The whole eastern horizon where the 

 tails meet and where the zodiacal light is was suffused 

 with a dim and indefinite glow, which was particularly 

 noticeable on May 18—19 and 20-21. This glow was not 

 so definite in boundary as the zodiacal light. W'hen the 

 comet v/as seen on the evening of May 20 we were sur- 

 prised to see it had the ordinary tail pointing away from 

 the sun as usual. It had been noticed for several days 

 that in the neighbourhood of the sun the sky was not 

 so blue as usual, but this was the case even a week before 

 the transit, and is probably merely a meteorological pheno- 

 menon. This brief summary of the facts will suffice here ; 

 the observations in detail will be published elsewhere. 



We have now to explain the reason why the earth did 

 not pass through the tail of the comet, and why the tail 

 broke up so that some of it was left in the morning sky, 

 where it remains, and is slowly losing its luminosity, and 

 some (or another tail) appeared in the evening sky. It 

 IS well known that a comet under the sun's radiant action 

 (I do not attempt to define it more closely) expels corpuscles 

 towards the sun which the sun repels, and these luminous 

 juscles form the tail. This process goes on even when 



- in the case of Halley's comet) the distance between 

 the comet and the sun exceeds the distance of the earth 

 from the sun. If the nearer planets do not show tails it 

 IS because these corpuscles have been shed by the planets 

 ages ago. In short, a comet and a planet under the 

 radiant action of the sun, and the sun itself, all repel 



NO. 2120, VOL. 83] 



Ox the morning of May 19, at between 4 and 4.30 a.m. 

 standard Indian time (55 hours from Greenwich), the tail 

 of Halley's comet could be seen stretching as far as the 

 Milky Way near Sagittarius, if not beyond. The tail was 

 much fainter than it had been two or three days previously, 

 but was still quite distinct. 



In the constellation Sagittarius, however, a dark band, 

 like a shadow, stretched diagonally right across the tail 

 upwards from east to south at an acute angle of about 

 20° to 30° with the direction of the tail. The edges of the 

 band were approximately straight and parallel, and the 

 width of the band was perhaps two or three degrees. 



No luminosity could be noticed within the band. 



If, as seems probable, the tail was then entering into the 

 shadow of the earth, it would appear that at any rate 

 the major portion of the light of the tail was light reflected 

 from the sun. A. S. Hemmy. 



Government College, Lahore, Punjab, May 26. 



The Term " Radian " in Trigonometry. 



Nature of April 21, containing Mr. Thomson's letter, 

 has just reached me, and I hasten to say that, had I known 

 that his father had ever claimed to have originated the 

 word " radian " I should, of course, have mentioned the 

 claim in my communication to Nature of April 7. As a 

 matter of fact. Prof. Thomson never did so in my presence, 

 and he certainly knew shortly after he came to Glasgow 

 that I had on my own initiative proposed the word, and 

 had made use of it for some j'ears. One day when I met 

 him accidentally he told me that he had found a college 

 student who had been a pupil of mine using the word 

 " radial " for a unit-angle, and that, while agreeing with 

 me as to the need of such a word, he had doubts as to 

 the suitability of the terminal syllable. My reply, as may 

 be guessed from my recent communication, was that 

 " radial," " radian," " rad," had all something to be 

 said for them, and I referred him to my letter to Nature 

 dated .April 4, 1870. On at least two subsequent occasions 

 we spoke of such things, and he supported the termination 

 -an in this particular case, because of a supposed analogv 

 with the geometrical term " median." All this, you will 

 see, does not preclude the possibility of an independent 

 origination of the term by him in July, 187 1, as stated by 

 Mr. " Thomson, and I therefore regret that here there is 

 no chance of me having the satisfaction of seeing the 

 printed word in the Calendar of Queen's College, Belfast, 

 for 1873-4. 



May I direct attention to the fact that in justification 

 of his letter Mr. Thomson unfortunately represents me as 

 saying that it was in 1874 that " the word was finally 

 adopted "? This is quite incorrect. If he will kindlv 



