June 25, 1874] 



NATURE 



149 



small mast with a halliard fitted on the port quarter- 

 deck. I saw that it was useless to attempt to nodce the 

 first moment of the entry of Venus, for I did not want to 

 fatigue myself and run the risk of not being able to ob- 

 serve the total immersion. Indeed, I had sufficient trouble 

 to fix the sun, on account of the movement of the ship. 



" When Venus had half entered, or nearly so, on the 

 disk of the sun, which I recognised by my reflecting 

 quadrant, I attached myself, so to speak, to the telescope 

 of 1 5 ft. to try to catch the moment of total entry. As my 

 watch was none of the best, and as I could not take the 

 height of the sun precisely at the moment when Venus 

 appeared to me to be totally immersed, it occurred to me 

 to make use of the sand-glass, by means of which the 

 way of the vessel was measured, and I had by my side a 

 man well up to turning the glass at the instant in such a 

 way that it was impossible to have an error of more than 

 a quarter of a second each time. 



'■ The weather having become overcast, and the rain 

 having shown itself, I did not think it would be possible 

 to notice the e.\it of Venus. Consequently I did not 

 cause the mast to be changed, as 1 ought to have done, 

 for we had tacked since half-past i r. 



'' At 2 o'clock it cleared a little, and shortly after the 

 weather cleared so that I could see Venus very distinctly 

 with my green objective, and wiihout the help of any 

 other coloured glass, and I was not incommoded. 1 saw, 

 from this observation, that it was not impossible for a per- 

 son used to the movement of a vessel, and accustomed to 

 the use of large instruments, to observe, especially when 

 the sea ii calm, the immersions of the satellites of Jupiter 

 wiih a telescope of 12 or 15 fc, which would have a large 

 field, and to determine the time of those immersions in 

 the above manner ; fori believe myself safe in asserting that 

 I did not make from them from 15 to 20 seconds in time 

 of error on an immersion of the first satellite of Jupiter." 



The observations made under these extraordiniry cir- 

 cumstances, give for the total immersion of Venus, Sh. 

 27m. 56.^5. ; the commencement of the exit, 2h. 22m. 53s. ; 

 the total exit, z'a. jSm. 52AS., which gives for the duration, 

 6h. lom. 55:','s., and for the time taken by the diameter to 

 cross the limb of the sun, 15m. 59s. As M. dc Seligny had 

 observed at the Isle of France the exit of Venus, Le 

 Gentil formed, for the meridian of his observation, 

 88° 20' 15". The log-book gave 87° 14' o". 



As there was to be another transit of Venus on June 

 3, 1769, Le Gentil resolved to spend eight years in the 

 southern hemisphere in wait ibr it. He had the devotion 

 to carry this resolution into effect, spending his time in 

 makmg a series of curious and interesting observations in 

 the Mascarene Islands, Madagascar, Marianne Islands, 

 the Philippines, and the coasts of India. He had fixed 

 on Manilla as his place of observation, and reached 

 it about August 1S66, but he was ordered to return to 

 Pondicherr\-. By what must seem a cruel fatality, this 

 patient devotee of science, when the day of the Transit 

 arrived, fi)und his view of the sun completely shut out by 

 clouds during the whole phenomenon, although for many 

 days previous the sky had been cloudless. On the other 

 hand, two friends whom he had left at Manilla were fortu- 

 nate enough to witness the transit without obstruction. 

 Le Gentil died on October 22, 1792. 



0\' THE TEMPORARV fADING OF SOME 

 LEAVES WHEN EXPOSED TO THE SUN 



FOR some time past I have taken much interest in this 

 subject, since it at first seemed to indicate that 

 chlorophyll in living plants could be decomposed by light 

 in the same manner as when dissolved out from them by 

 alcohol or other solvents. It also seemed to agree with 

 the fact which I had established by comparative quantita- 

 tive analysis, that leaves grown much exposed to the sun 

 contain a relatively less amount of chlorophyll than those 

 somewhat more shaded, in some cases e^-en only one- 

 third the quantity. My attention was first called to a 



diurnal change in the colour of a kind of moss commonly 

 grown in hothouses, by Mr. Ewing, of the Sheffield 

 Botanical Gardens, and subsequently to a similar change 

 in a tropical species of raaiden-hair fern, by Dr. Branson 

 of Baslow. In botli cases the colour of the fronds, after the 

 darkness of night, was deep green, but after exposure to 

 the bright sun of day it was a far paler and whiter green, 

 which was again restored by the subsequent absence of 

 light. I was particularly anxious to ascertain whether this 

 change was due to a diminution in the amount of chloro- 

 phyll, but was unable to detect any well-marked difference 

 by careful comparative quantitative analyses. I therefore 

 came to the conclusion that, at all events in the case of 

 the moss, the change in colour was due to some sort of 

 mechanical alteration in the structure of the fronds, but 

 did not examine the question more fully. The true ex- 

 planation appears to be that adopted by Prillieux, who 

 describes his observations in Comptes Rcndiis, t. Ixxviii. 

 p. 506. According to him and to the previous experiments 

 of Famintzin and Borodin, exposure to bright light causes 

 both granular and amorphous chlorophyll to collect to- 

 gether at the sides of the cells, instead of being more 

 evenly distributed. The result of this is that a much 

 larger relative proportion of white light is reflected, and 

 the leaves or fronds appear of a paler and whiter green. 

 These conclusions are thus in perfect agreement with my 

 own quantitative analyses, and we may, I think, look 

 upon this combined evidence of two independent methods 

 as furnishing a satisfactory explanation of the greater 

 part, if not of the whole, of the temporary change in 

 colour. H. C. SORP.Y 



THE COMET 

 A FTER a very unusual amount of difficulty in the 

 -^^ determination of the orbit I have succeeded in de- 

 ducing a set of parabolic elements which appear to 

 possess considerable precision. They are as follows : — 

 Perihelion passage, July, 8-83652 Greenwich M.T. 



Longitude of Perihelion ... 271 3 5 r; ( Mean equinox 



,, Ascending node ... 118 43 25'5 j July o 



Inclination to ecliptic ... 66 21 160 

 Log. Perihelion distance ... 9'829S7i9 

 Motion direct. 

 Ourlast observation, a very good one, gives this position: — 

 June 22, at loli. 4m. 21S. M. T. at Twickenham. 

 R.A. ... 7h. 21m. 58-055. 

 D. ... +6S=9'34'-5 

 which compared with the above orbit (parallax and aber- 

 ration allowed for) shows only the following insignificant 

 diiferences — in R.A. —2" ; in D. -j- 14". 



This close agreement with parabolic motion is not 

 favourable to identity of the comet with that of 1737, not- 

 withstanding similarity of elements, but we must look to 

 observers in the southern hemisphere to enable us to 

 decide this point. The comet may certainly be there 

 observed till October or November in the Antarctic cir- 

 cumpolar heavens. 



The subjoined ephemeris will suffice to indicate the 

 course of the comet, while it continues visible in our 

 latitudes : — 



At Greenwich— J//rt'/«X'/'/- 



R.A. N.P.D. Distance. '"tenslly 

 01 hg >t. 



h. m. „ , 



June 25 7 273 22 33 0-S16 2-4 



27 7 30-0 23 II 0-769 2 8 



29 7 33-7 24 3 0-721 3-3 



July I 7 30-5 25 10 0-673 3'9 



3 7 39'i 26 34 0-624 4'0 



5 7 41 '3 2S 24 0-575 S'5 



7 7 43-: 30 46 0-528 6-6 



y 7 44'8 33 48 0-4S2 7-9 



II 7 46-2 37 39 0-437 y-6 



13 7 47'5 42 30 o'3yo ii'S 



15 7 486 48 33 0-359 13-7 



