November 24, 192 1] 



NATURE 



403 



of air. Not only is this in agreement with observa- 

 tion, but I find the coefficient of extinction of light 

 due to scattering 



^i>-)v-; 



closely represents the observed transparency of pure 

 water in the region of the spectrum where there is no 

 -elective absorption. Work is now in progress testing 

 "he formula in the case of other liquids. 



It is clear that an application of the same idea 

 of local fluctuations of optical density and of Debye's 

 theory of the thermal movements in solids would give 

 the theoretical scattering power of transparent crystals 

 for ordinary light. This is also being tested. 



C. V. Raman. 



210 Bowbazaar Street, Calcutta, October 15. 



The Tendency of Elongated Bodies to Set in the North 

 and South Direction. 



The letter from Sir Arthur Schuster in Nature of 

 October 20 last requires amplification and amendment 

 in one particular. The setting tendency of an 

 elongated body depends upon its method of support. 

 If suspended, with the centre of gravitv not free to 

 rise and fall, it is at its "lowest" position when 

 lying on the equipotential section of maximum radius 

 of curvature, i.e. tends to set east and west. A 

 floating body, on the other hand, where the centre 

 of gravit\^ is free to rise and fall, is at its lowest 

 when lying on the equipotential of minimum radius 

 of curvature, i.e. north and south. The whole matter 

 is fully discussed in an article by Mr. W. D. Lambert, 

 of the United States Coast and Geodetic Survey in 

 the American Journal of Science for September last. 



The tendency of the rod of a torsion balance to 

 set east and west was pointed out by Baron Eotvos 

 in one of his early papers, probablv one of those pre- 

 sented to the International Geodetic Conference, but 

 :, • I am not able to lay my hands on the exact reference. 



E. H. Grove-Hills. 



Ophion luteus. 



It is a quite common experience to see Ophion 

 luteus fly into houses at night attracted by light. I 

 have myself captured at least half a dozen specimens 

 that had in a single hour flown into a room in that 

 way. Not long ago Dr. James Waterston, at my 

 request, dissected a fresh female specimen, and found 

 in it a poison gland, reservoir, and duct similar in 

 character to those recorded as being present in certain 

 other species of Ichneumonidae. 



M. R. du Buysson, in a paper (Rev. d'EntomoIogie, 

 vol. II, p. 257, 1892) which I have only recently 

 seen, states that he had often been stung by Ichneu- 

 monidae of the Ichneumon, Pimpla, and Ophion 

 groups ; but however much poison may have been 

 injected, the pain and inflammation produced by the 

 sting, he says, lasted only a short time. He dis- 

 sected a large number of specimens belonging to all 

 the groups of Ichneumonidae, and always found one 

 or several poison glands present. 



The larvae of Ophion luteus appear to be parasitic 

 in the caterpillars of many different species of 

 Lepidoptera ; but it would appear to be the case also 

 that the female does not pierce the skin of the cater- 

 pillar to lay her eggs inside. She is said to lay them 

 on the skin. That point probably needs confirmation. 

 But however that may be, it seems to me incredible 

 that the female wou'd mistake the arm of a voung 

 lady for a caterpillar. In M. du Buysson 's case, the 

 insect had always been held in the hand or otherwise 

 irritated before it attempted to sting, and this seems 

 NO. 2717, VOL. I08I 



to be the general experience. He was never puzzled 

 to divine the purpose of the sting, regarding it, no 

 doubt, simply as an act of self-defence ; and that is 

 the explanation which I would venture to suggest in 

 reply to Sir Herbert Maxwell's letter in Natx:re 

 of November 10. C. J. Gahax. 



Natural History Museum, S.W.7, 

 November 14. 



Ix respect of Sir Herbert Maxwell's letter in 

 Nature of November 10 on Ophion luteus, Linn., 

 may we hof>e that the insect caught flagrante delicto 

 was preserved in order that the species might be 

 placed beyond a doubt, these large red Ophionidae 

 being almost impossible to differentiate at a glance? 



Ophion luteus is apparently a nocturnal insect. I 

 have observed it at night hunting for Dianthoecia 

 lar\-ae. The species is credited with a long list of 

 hosts, chiefly Noctuid moth caterpillars. 



Almost all Ichneumonidae will "sting" or attempt 

 to if handled, the males included (though, of course, 

 morphologically incapable), but that any member 

 should make an attack unprovoked is most unusual. 

 O. luteus and allied species are extremelv bad- 

 tempered — a fact which hampers work with them in 

 confinement, as they repeatedly "sting" potential 

 hosts to death without attempting to parasitise them. 

 Perhaps this irritability was the cause of the unusual 

 attack. R. Stentox. 



Pathological Laboratory, Ministry of Agricul- 

 ture, Harpenden, November 19. 



Sex-manifestation and Motion in Molluscs. 



I DO not wish to prolong the discussion up>on sex- 

 difl'erentiation and mode of life, though I venture to 

 think that Dr. Orton's reply in Nature of Novem- 

 ber 3 to my letter in the issue of October 13 leaves 

 several questions very much as thev were before. 



On many points I find myself in agreement with 

 Dr. Orton, though I consider that the incidence of 

 sex-differentiation in the Mollusca does not exhibit 

 that general correlation with an active habit de- 

 manded by Dr. Orton's hypothesis. I quite agree, 

 as I said in my previous letter, that many forms 

 originally considered dioecious viay be monoecious ; 

 but I think it is for Dr. Orton to prove this, and I 

 shall await his demonstration with interest. I would 

 like to point out, however, that it will not be enough 

 to show sex-change (a turnover from maleness to 

 femaleness, or vice versa). The Implications of Dr. 

 Orton's hypothesis entitle me to demand from him 

 something in the nature of permanent hermaphrodite 

 forms. G. C. Robsox. 



Natural History Museum, S.W.7, 

 November 14. 



Sinistral Limnaea peregra. 



Last year_ I started a breeding experiment with two 

 pairs of sinistral Limnaea peregra given me by Mr. 

 J. W. Taylor, of Leeds. The first two generations 

 have not come out on any plain plan, and it is neces-. 

 sary to carry them further if the mode of inheritance 

 of this very rare form of one of our commonest fresh- 

 water snails is to be worked out. But the voung 

 have now quite outgrown the possibilities of mv 

 establishment, and if anyone would take over some 

 of them and breed them out (which is quite simple, as 

 they want little attention) thev would be doing me a 

 service. ' A. E. Boycott. 



University College Hospital Medical School, 



L'niversity Street, Gower Street, W.C.i, 



November 7. 



