February 8, 1906] 



NA TURE 



34i 



Scintillations produced by the Electronic "j3-Rays'' 

 emitted by Radium. 

 As the j3 particles emitted by the radio-active elements 

 are analogous to the a particles, inasmuch that they may 

 be considered as parts of the disintegrated atom, and not 

 in any sense true rays, I have been conducting some experi- 

 ments with the view of ascertaining if, in any circum- 

 stances, their action upon fluorescent screens caused re- 

 cognisable scintillations. It will be obvious that if the 

 radium used be placed too near the screen, the effects of 

 the combined /3 and y rays will produce a fluorescence 

 sufficiently vivid to mask any scintillations due to the 

 individual electrons which compose the /3 stream. In order 

 to diffuse this action and allow the /3 particles to separate 

 to such a degree that the flashes produced shall not over- 

 lap, recourse may be had to three methods : — 



(1) Increasing the distance between the radium and the 

 screen. 



(2) Making the stream pass through material offering 

 resistance to its passage. 



(3) Reducing the quantity of radium used and " diluting " 

 the action by mixing it with non-active substances. 



I have tried these methods separately, and in each case 

 have obtained results which were fairly satisfactory con- 

 sidering the difficulties attending this class of observation. 



In these circumstances the screen, when viewed with a 

 lens having a focus of about half an inch, was seen to be 

 fluorescing with a faint glow, which was, without doubt, 

 of an unsteady and flickering character. 



The phenomena involved are delicate and difficult to 

 observe, requiring the best possible conditions. It is very 

 important that the eyes are thoroughly rested before the 

 observation is made, but the fact that the glow is flicker- 

 ing and strongly suggestive of scintillations is, in my 

 opinion, beyond dispute. After trying various combinations 

 of the above methods, I have obtained results sufficiently 

 decided to justify the statement that the screen was lit 

 up by scintillations properly so called. A screen of zinc 

 sulphide, so very excellent for viewing the scintillations 

 produced by the a particles, does not give satisfactory 

 results for the /3 stream. 



Willemite and barium-platino-cyanide are both fairly 

 good, the latter being rather the better of the two. I have 

 obtained the best results with an ordinary X-ray screen. 

 Having enclosed about 15 or 20 milligrams of radium- 

 barium-bromide (1 per cent.) in a small glass tube, I placed 

 over it a sheet of mica. Over this was placed a sheet of 

 cardboard, and above this again, at a distance of about 

 half an inch, was the screen. When the lens was focused 

 on the screen, a dim fluorescence, due to the y rays, was 

 seen as a sort of background, on which were visible faint 

 nebulous scintillations coming and going in a manner very 

 similar to the scintillations produced by the a particles on 

 a zinc sulphide screen. C. W. R. 



February 2. 



The Effect of Food on the Colour of Moths. 



In a very interesting paper published in the Journal of 

 Economic Biology (1905, No. 1), Mr. W. E. Collinge 

 describes and figures a remarkable series of specimens of 

 the magpie moth [Abraxas grossulariata, Linn.), obtained 

 as the result of raising the larvae on lettuce, the ordinary 

 food being currant. The specimens all differed from the 

 type in the direction of great loss of markings, the most 

 extreme one representing the aberration known as dohrni 

 or deleta. The same sort of effect has been produced on 

 the tiger moth by G. Koch, as a result of feeding the 

 larvae on lettuce ; and a good account of various experi- 

 ments of the same kind is given by Dr. Vernon on 

 pp. 288-q of his work on " Variations in Animals and 

 Plants " (1903). 



The effect produced in the cases cited may be regarded 

 as a sort of compulsory mutation, though we do not know 

 whether it could in any case be inherited in such a manner 

 that it would remain constant under different conditions. 

 If the normal maculation, which has existed for count- 

 less generations, can be transformed in a single one by a 

 new food-plant, it is not likely that the alternate type 

 can be any better fixed. 1 The species must be regarded 



1 In this connection, cf. "The Principles of Heredity," by G. Arcbdall 

 Reid, 2nd edition, Appendix A, pp. 355-356. 



NO. 1893, VOL - 73~\ 



as potentially dimorphic, indeed, polymorphic, for other 

 quite different aberrations occur. There arises, however, 

 an interesting possibility. Mr. Collinge found that the 

 insects were raised with difficulty on lettuce, and he 

 doubted whether it would be possible to raise three suc- 

 cessive generations exclusively on that plant. Suppose, 

 however, that among many which fed on lettuce (or any 

 unwonted food) a few were able to survive, and con- 

 sequently a lettuce-feeding race became firmly established. 

 Such a race would show the same marked differences from 

 the type which appeared in the first generation, and it is 

 not unlikely that at length it would be as difficult to get 

 its members to live on currant as it originally was to get 

 the currant-feeders to survive on lettuce. We should then 

 have a lettuce-feeding form, very easily distinguished from 

 the currant-feeding one, and nobody would hesitate to call 

 it a distinct species. If it absolutely refused to feed on 

 currant, the peculiar markings would be as fixed as in 

 any species known. 1 



These suggestions appear to have the more force from 

 the fact that some of the lettuce-fed examples strongly 

 recall Abraxas sylvata, which feeds on elm, and is 

 universally considered distinct. This A. sylvata and its 

 allies form a group of closely allied races in the Palasarctic 

 and Oriental regions, and it would be extremely interesting 

 to ascertain whether these several forms have different 

 food-plants, and whether by changed conditions they can 

 be derived from one another. Many years ago I had 

 occasion to tabulate these forms, using the material in the 

 collection of the British Museum, and in the hope that 

 the matter may be taken up by some eastern entomologist 

 I give here the brief table I made : — 



A. — Markings strongly developed. 



(a) Expanse about 34 mm. ; Europe, Siberia, &c. . . . 



sylvata, Scopoli. 

 (a) Markings stronger than type or darker ; brown 

 anal blotch more reduced; Japan, &c. . . 

 sylvata var. intensa, Warren. 



(b) Larger ; nearly always more than 40 mm. expanse, 

 (a) Markings strong and dark. China and Japan 



. . . miranda, Butler. 

 ($) Markings paler. India . . . leopardina, Kollar. 



(c) Still larger ; expanse more than 50 mm. ; markings 



rather pale. 

 (a) Markings more suffused. N. China . . . 



plambeata, Warren. 

 (0) Markings less suffused. Silhet . . . illuminata, 

 Warren. 

 B. — Markings much reduced, but the brown blotches 

 remaining well developed. 



(a) Expanse more than 40 mm. ; markings rather more 



developed than in pantaria. India . . . paucino- 

 tata, Warren. 



(b) Expanse more than 35 mm. ; Europe . . . pan- 



taria, Gn. 



(c) Expanse about 30 mm. ; markings still more re- 



duced. Europe . . . seasonal form calaria, Gn. 



Since the physiological adaptation to the new food-plant 

 is not really connected with the change of colour or 

 maculation, it may frequently take place without any 

 externally visible signs, or such signs may only arise 

 after a long period. In this way we get " physiological 

 species," which are no doubt more numerous than is 

 generally supposed. T. D. A. Cockerell. 



University of Colorado, Boulder, Colorado, U.S.A., 

 January 18. 



A Correction. 



In the review of Prof. Fr. Czapek's " Biochemie der 

 Pflanzen " (Nature, vol. lxxiii. p. 192) I mentioned that 

 I missed a certain paper by Schjerning. The author's 

 name should have been Weis. 



I also overlooked the reference to a paper by Cornevin. 



An index of authors' names would enhance the utility of 

 the book. F. Escombe. 



1 Pictet, quoied by G. A. Ri 

 a new food-plant, certain buttei 

 to the original type. Of cout 

 this does not take place, but ej 

 indicated. 



id. reported that after several generations on 

 flies which had at first been modified reverted 

 se, the case I have imagined is one in which 

 periment is needed to test the possibilities 



