672 



NATURE 



[February 18, 19 15 



bromine — the one on which the most trustworthy 

 data have been obtained. Instead of the fluorescent 

 radiation in the particular case carrying away 50 per 

 cent, of the energy of the primary radiation specially 

 absorbed, experiments indicate about 47 per cent, and 

 approximately an equal value for the corpuscular 

 radiation. But there is evidence that with elements 

 of higher atomic weight a limiting value of about 

 50 per cent, would be obtained. The indication of 

 such a limit gives strong support to the whole theory. 

 On the other hand, from an element of low atomic 

 weight, the experimental value for the fluorescent 

 radiation comes below 30 per cent., and there are indi- 

 cations of even lower values. 



It would, however, be remarkable if such a simple 

 theorv gave perfect agreement for all elements, and 

 correspondingly all X-radiations. The facts indicated 

 appear of fundamental importance ; deviations — real 

 or apparent — will receive investigation and discussion 

 later. 



Other important conclusions based on the investiga- 

 tion are that the absorption by an atom is not neces- 

 sarily in whole quanta of the primary radiation ; we 

 have evidence of absorption of primary radiation in 

 quantities of any magnitude between one and two 

 quanta of the primary radiation, or just possibly in 

 tractions of one quantum. 



The transformation of primary radiation into 

 fluorescent radiation in certain cases at least is accom- 

 panied by little, if any, appreciable loss of energy 

 within the atom. 



The energies of X-radiations differing widely in 

 penetrating power are approximately if not accurately 

 proportional to their total ionising powers. 



Details of these investigations will be published 

 shortly. C. G. Barki.a. 



Physical Laboratory, The University, 

 Edinburgh, February 8. 



The Green Flash. 



So much has been written about the green-ray at 

 sunset that I am somewhat diffident about adding 

 anything. But as I find myself unable to accept the 

 orthodox explanation of the phenomenon usually seen 

 I write this note. This phenomenon, as seen by me 

 on several occasions during the last summer on my 

 way to Australia, always consisted in the last seg- 

 ment of the red sun before disappearance becoming a 

 bright green (without any transition through inter- 

 mediate tints) ; this green was as nearly as could be 

 judged the complementary to the red of the sun itself. 

 On one occasion I shut my eyes immediately after 

 the green tint appeared, and it remained visible. 

 There could be no doubt that what I saw was the 

 purely subjective after-image of the disappearing seg- 

 ment of the sun. Of course, if this is so, it should 

 be easy to set up a laboratory experiment to imitate 

 the natural phenomenon ; and on returning I asked 

 Mr. E. Talbot Paris, research student in this depart- 

 ment, to arrange an experiment in illustration. An 

 eccentric hole was made in a disc mounted on an 

 axle. Red glass or gelatine film was fixed over the 

 hole, and a bright light placed behind illuminated the 

 film and produced thereby a miniature sun, which by 

 slow rotation, could be made to " set " behind an 

 interposed card. At the instant of setting, the arti- 

 ficial sun exhibited an exact reproduction of the 

 phenomenon of the green-ray. It was easilv possible 

 in this way to obtain a red-ray using a green sun, 

 or a blue-ray with a yellow sun, and _so on. 



It is easy to give the rationale of the effect. The 

 positive light gradually diminishes as the artificial 



NO. 2364, VOL. 94] 



sun passes below the horizon ; and it only requires 

 a little adjustment of the rate of disappearance in 

 order that the negative after-image excited at a pre- 

 vious instant when the segment was brighter, shall 

 overpower the simultaneous weaker positive image of 

 the remaining segment itself. 



It would not be fair for me to dogmatise and assert 

 that this is the only phenomenon which comes under 

 the head of the green-ray. But it is certainly the only 

 one which I succeeded in seeing ; and it must always 

 be present even on the possible rarer occasions when 

 colour changes arising from dispersion are also 

 evident. It is certainly also what many others saw. 

 At the same time, it must be added that the pheno- 

 menon as observed by different persons, even on the 

 same night, was so variously described as to lead one 

 to suppose that the subjective element is sometimes 

 present to even a greater degree than is implied in the 

 above note. Alfred W. Porter. 



Physical Department, University College, 

 London, February 7. 



Trenching Ground and Spraying Potatoes. 



In the notice in Nature, of February 4, of 

 the fourteenth report of the Woburn Experi- 

 mental Fruit Farm it is suggested that the negative 

 results obtained by us in bastard trenching might 

 have been different had we experimented on vegetables, 

 instead of fruit trees. No doubt the suggestion is 

 correct ; and a chance observation last year gave 

 a striking illustration in point. Brussels sprouts were 

 grown in a piece of ground partially occupied by trees ; 

 the ground had all been dug, but there were four 

 patches of about four square yards each where it had 

 been practically trenched, by the removal of trees and 

 the digging out of their roots. In each of these 

 patches the sprouts were two to three times larger 

 than those in the intermediate dug ground. Universal 

 experience indicates that a good depth of rich soil is 

 essential for successful vegetable growing; this can 

 only be obtained by trenching and liberal manuring, 

 and nothing in our results should be taken as dis- 

 countenancing such a practice. 



It is also suggested that we should accumulate results 

 on potato spraying to see whether such treatment pays 

 on the average. We are doing so, and those already 

 obtained are nearly sufficient for the purpose. They 

 extend over eight seasons, and are on a fairly large 

 scale, though the diversity in conditions, adopted for 

 other reasons, renders it somewhat difficult to deduce 

 a fair average from them. As it stands, this average 

 is 78 per cent, increase on the weight of sound tubers 

 as a result of spraying. Putting the average yield at 

 7 tons to the acre, and the net price realised at 3Z. los. 

 per ton, the value of the increment will be il. 18s. 

 Two sprayings would cost, for materials, labour and 

 use of plant, about 185. to iZ. 13s., according to the 

 substance used, and this would leave a margin of 

 profit of from 55. to 20^. per acre. 



Spencer Pickering. 



Harpenden, Herts. 



Early Representations of the Giraffe. 



The discussion in Nature during the past year 

 concerning the first mention in literature of the 

 opossum and kangaroo has suggested a similar inquiry 

 with regard to certain other well-known animals of 

 the New and Old World. 



When one examines into the sources whence were 

 derived the illustrations in early printed books on 



