28 



NA rURE 



[May 14, 1896 



The meteor of April S was directed from a radiant in the 

 eastern limits of Virgo, and not far from Spica. A fireball was 

 seen on March 16 last, which was probably from the same 

 radiant, as the paths converge on the point 205' - l8'. This 

 region is the centre from which many fireballs and ordinary 

 shooting stars are directed in April and other months, as the 

 following table will prove : — ■ 



The mean of the twenty-six positions is 209° - 9°. 



The fireball of April 12, 1896, came from a radiant in the 

 N.W. sky at 50° + 42°. The large meteor of April 22, 1894, 

 had a similar radiant [Ohsoi'alory , June, 1S94), and the same 

 may be said of the fireball of March 9, 1875. 



Bristol, April 27. \V. F. Dexning. 



Becquerel and Lippmann's Colour Photographs. 

 I WISH to raise a point in connection with the optics of 

 photochromy, which was not touched upon at the recent dis- 

 cussion at the Royal Society. The photochromatic spectra pro- 

 duced by the earlier workers, and especially by E. Becquerel 

 about 1S50, have long been known and have always appeared to 

 be very mysterious to those who have repeated the experiments. 

 Wlien Prof. Lippmann's success with the interferential method 

 was made known some five or six years ago, and his first results 

 exhibited in this country, many of those who were acquainted 

 with the previous methods of producing coloured spectra by 

 direct impression came to the conclusion that all the earlier 

 workers had unconsciously been producing the Lippmann effect. 

 This supposition was not unreasonable. In Becquerel's method, 

 for instance, which gave the most brilliant effects, the sensitive 

 film of violet chloride is produced on a surface of metallic silver, 

 and is thus backed by the necessary reflecting surface. Even 

 when the colour sensitive chloride is on paper, as in the still 

 earlier experiments of Robert Hunt and .Sir John Herschei, it is 

 not unreasonable to suppose that the bounding surface of the 

 paper and silver haloid reflects sufficiently well to produce the 

 necessary interference. At the discussion following I'rof. Lipp- 

 mann's paper. Lord Rayleigh raised the question whether the 

 earlier and later results were not due to the same cause, but there 

 seemed to be an impression that the Becquerel and Lippmann 

 effects were produced by different causes. P"or my own part, I am 

 bound to confess that the reasons assigned for arriving at this 

 decision still appear to be inconclusive. The main points which 

 have been allowed to prevail are that the Becquerel photographs 

 cannot be fixed, that they appear of the same colour at whatever 

 angle they are viewed, and that they appear of the same colour 

 by transmitted and by reflected light. The fact that these photo- 

 graphs cannot be fixed is easily explained if we bear in mind that 

 the silver salt, is not embedded in a vehicle, as in Lippmann's 

 process, and that there is consequently nothing to hold the 

 lamina: apart at the correct intervals when the fixing solution 

 has done its work. The other points are less easy to explain ; 



NO. 1385, VOL. 54] 



but it may be suggested that the difference is here due to the 

 earlier experimenters having used coarse-grained films, in which 

 the silver haloid ])articles are sufficiently large to scatter the 

 colours produced in the film by the laminated structure of the 

 alternating planes of decomposition and no decomposition. The 

 question is a purely physical one, and may be put into the 

 following form : — If the Lippmann eftect is produced in a coarse- 

 grained instead of in a transparent film, would not the Becquerel 

 results be obtained ? If physicists can answer this in the afhrm- 

 ative, the difliculty of supposing that similar results can be 

 obtained by totally different causes would disappear. 



R. Mei.I)Oi..\. 



Aquatic Hymenoptera. 



U.N'DER the title " On Two Aquatic Hymenoptera, one of 

 which uses its Wings in Swimming," Sir John Lubbock, Bart., 

 read a paper before the Linnean Society, May 7, 1863, therein 

 describing two most extraordinary insects, which he named 

 Polynema imtaiis and Prestwichia atjuatka. 



Last year I had the good fortune to obtain a large number 

 of both sexes of the first named, which, after most critical 

 microscopic examination, I identified as belonging to llaliday's 

 Caraphractiis iiintiis, the unique characteristic of the ''^ kcdcd 

 metathorax " placing the matter beyond a doubt. The late I'rof. 

 Riley, to whom I had the pleasure of showing specimens, fully 

 confirmed my opinion, as also did Mr. Charles Waterhouse. 



The life-history of any of these minute Hymenoptera is not 

 worked out in one season — very far from it ; and since last year 

 I have steadily followed up the chain of facts, my efforts being 

 again rewarded by finding this most exquisite Ilymenopteron 

 this season within twenty miles of London. 



Encouraged by my success, I continued my search for some 

 hours at a small pond, and at last captured two female sf)eci- 

 mens of the long-lost-sight-of Prestwichia aqiiatica (Lubbock), 

 which has not been recorded since its first capture by Sir John 

 Lubbock in 1862 — thirty-four years ago ! 



The two specimens (and /) have scarcely taken any rest 

 since their capture yesterday morning. May 4 : but they have 

 been constantly running or paddling under water, never once 

 having been to the surface. When I first put them into the 

 tank, they had the^cir/t'.f^ difficulty in forcing their way through 

 the film ; but as soon as that was accomplished, they moved 

 about with their legs, as propellers, far more rapidly than did 

 Caraphractiis ciiicliis with its wings. 



I am looking forward to cajituring the male Prestwichia 

 aqiiatica, which has not yet been recorded by any entomologist. 



21 Manor Gardens, HoUoway, N. Fkeii. Enock. 



Dalton's Atomic Theory. 



In the review of "A New View of the Origin of Dalton's 

 Atomic Theory," published in your issue of April 16, your 

 reviewer, in summing up the evidence as to the origin of the 

 atomic theory, makes an omission of such importance that it 

 cannot be allowed to pass unchallenged. He attaches great 

 weight to Thomson's statement that in 1804 Dalton himself 

 informed him "that the atomic theory first occurred to him 

 during his investigations of olefiant gas and carburetted hydrogen 

 gas." Now these researches, as pointed out by your reviewer, 

 were begun in the summer of 1804, a date which is assigned to 

 them by Dalton himself, and is confirmed by the entries in his 

 laboratory note-books of the time ; so that Thomson's statement 

 amounts to saying that the atomic theory first occurred to 

 Dalton in the sttminer of 1S04. This conclusion ajipears to us 

 to be entirely discredited by the fact that several detailed tables 

 of atomic weights and lists of atomic symbols, which are dated 

 Septeiiilier iSoj, occur in Dalton's laboratory note-books, one of 

 these tables being reproduced in facsimile at p. 28 of the work 

 under review, but not referred to by your reviewer. 



It must be remembered that Thomson's account of the origin 

 of Dalton's theory was first published in his " History of 

 Chemistry" (vol. ii. p. 291) in 1S31, no less than twenty-seven 

 yeais after his visit to Dalton had been paid. Moreover, in 

 1S50, after the lapse of another nineteen years, he gave a second 

 and totally different account of the origin of the same theory, 

 saying it was fiiunded on the analysis of protoxide and dentoxide 

 of nitrogen (Henry, " Life of Dalton," p. So). 



Tin; Authors, 



The question is whether Dalton was Iiril to apply the 

 Newtonian doctrine of atoms to the explanation of chemical 



