zte 



NATURE 



[November 17, 192 1 



Letters to the Editor. 



\The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.} 



Metallic Coloration of Chrysalids. 



I HAVE read JNlr. Mallock's letter (Nature, Novem- 

 ^^ 3> P- 302) on iridescent colours with the greatest 

 interest, but I cannot help thinking that in some cases 

 his statements are slightly misleading, Mr. Mallock 

 seems to imply that all the iridescent colours of the 

 animal world are due to some form of interference. It 

 is trlie that he restricts his statement to the cases he 

 h^s examined, but it seems safe to assume that if 

 many colours were due to other causes, Mr. Mallock 

 would have met with some of th^se among the many 

 hundreds of cases investigated during the twenty years 

 ill which he has prosecuted his researches. 



I cannot lay claim to having carried out such exten- 

 sive investigations, nor have 1 the training in physics 

 which gives so much weight to Mr. Mallock's words, 

 nevertheless I venture to urge that the phenomena 

 of insect iridescence (not to speak of the rest of the 

 animal world) include rtiany cases which cannot be 

 brought into the category of interference. These ex- 

 ceptions cannot be described now, but some have 

 appeared in Nature (September ;^o, October 7, and 

 October 14, 1920), and a fuller account is to be found 

 in Phil. Trans. Roy. Soc., B, vol. 211, pp. 1-74. 



It would be interesting to know how Mr. Mallock 

 explains the colours of the wings of those beetles, 

 bees, and dragon-flies which the late Lord Rayleigh, 

 Prof. Poulton, and myself have found would change 

 colour neither on pressure nor when immersed m 

 fluids under reduced pressure. Such beetles include 

 the rose beetle (Cetonia aurata), many Buprestids, and 

 other common insects, some of which Mr. Mallock 

 is sure to have examined. Then there are the golden 

 elytra of P. resplendens, which resist pressure and 

 change to magenta on being polished ; also the 

 numerous iridescent tortoise beetles, the colour of 

 which not only resists pressure, but even the removal 

 by polishing of the thick surface-laver of chitin without 

 altering its appearance. Among iridescent birds 

 Mandoul has found that peacock's feathers resist 

 pressure, and even hammering on an anvil ; the 

 bright-coloured feathers of Cotinga, which Haecker 

 and Meyer attribute to "blue due to the scattering of 

 small particles " by fane canals' (Porencandlen) in the 

 keratin, would, I presume, not be considered by Mr. 

 Mallock as true iridescent objects. 



Finally, there are the beetles to which Biedermann 

 first directed attention (e.g. H. africana). Whether 

 or not the cause of their colour is the same as in the 

 last case, sections of the elytra in the plane of the 

 wing are of nearly the same colour as the original 

 beetle. Sections at right angles to the elytra are the 

 same yellowish colour as the chitin. 



Mr. Mallock also asserts that the colours of birds 

 and insects are not, as has so often been said, due 

 to difTraction. I admit that I know of no butterfly 

 in which the principal colours are caused in this way, 

 but what of the pale Morphos and other insects which, 

 when the wing is partly turned, exhibit all the colours 

 of the spectrum superimposed on the ground colour? 

 These colours correspond exactly, in appearance and 

 angle, to those of the reolica diffraction gratings made 

 iq collodion from the wings. 



Personally, I am prepared to agree that the wings 

 of almost all iridescent Lepidoptera owe their colours 

 to interference, but it would be interesting to learn 

 NO. 2716, VOL. 108] 



Mr. Mallock's reasons for disagreeing with so eminent 

 an authority as Prof. Michelson, who is of the opinion, 

 on purely physical grounds, that all the colours of 

 insects are due to selective metallic reflection, with 

 the exception of the iridescent scale-bearing weevils 

 (e.g. the diamond beetle), the colours of which he 

 attributes to diffraction — a cause ruled out by Mr. 

 Mallock. 



I have referred only to the colours of insects and 

 birds, but it would be most interesting to know to 

 what forms of structure Mr. Mallock attributes such 

 striking examples of iridescence as are to be found 

 among the hairs of some mammals, the setae of many 

 marine worms, certain ferns and seeds, and many 

 brilliantly coloured Crustacea, some of which Mr. 

 Mallock must surely have examined. 



H. Onslow. 



3 Selwyn Gardens, Cambridge, November 9. 



The Softening of Secondary X-rays. 



A NUMBER of experimenters have noticed that when 

 a beam of X-r^ys or 7-rays traverses any substance, 

 the secondary rays excited are less penetrating than 

 the primary rays. Prof. J. A. Gray (Franklin Insti- 

 tute Journal, November, 1920) and the present writer 

 (Phil. Mag., May, 192 1, and Phys. Rev., August, 192 1) 

 have shown that the greater part of this softening is 

 not due, as was at first supposed, to a greater scatter- 

 ing of the softer components of the primary beam, but 

 rather to a real change in the character of the radia- 

 tion. My conclusion was that this transformation 

 consisted in the excitation of some fluorescent rays of 

 wave-length slightly greater than that of the primary 

 rays. Prof. Gray, on the other hand, showed that if 

 the primary rays came in thin pulses, as suggested 

 by Stokes's theory of X-rays, and if these rays are 

 scattered by atoms of- electrons of dimensions com- 

 parable with the thickness of the pulse, the thick- 

 ness of the scattered pulse will be greater than that 

 of the incident pulse. He accordingly suggests that 

 the observed softening of the secondary rays may be 

 due to the process of scattering. 



It is clear that if the X-ravs are made to come in 

 long trains, as by reflection from a crystal, the scat- 

 tering process can effect no change in wave-length. 

 On Gray's view, therefore, if X-rays reflected from a 

 crystal are allowed to traverse a radiator, the incident 

 and the excited rays should both have the same wave- 

 length and the same absorption, coefficient. If, on 

 the other hand, the softening is due to the excitation 

 of fluorescent rays, as I had suggested, reflected 

 X-rays should presumably be softened bv scattering 

 in the same manner as unreflected rays. An examina- 

 tion of the absorption coefficient of reflected X-rays 

 before and after they have been scattered should 

 therefore afford a crucial test of the two hypotheses. 



The double reduction in intensitv which occurs 

 when the X-ray beam is first reflected bv a crystal 

 and then scattered by the radiator made Gray's pre- 

 liminary attempts to perform this expyeriment unsuc- 

 cessful. In the September (1921) issue of the Philo- 

 sophical Magazine, however, Mr. S. J. Plimpton 

 describes a successful attemnt to measure the absorp- 

 tion of the K lines from rhodium and molvbdemmi 

 after being scattered by paraffin and water. He 

 observed no change in the absorption coefficient of 

 the ravs after being scattered by the paraffin. An- 

 parently his measurements were made on the secon- 

 dary rays at comparativelv small angles, and this, 

 together with the relatively long, wave-lengths em- 

 ployed, form the conditions under which the least 

 , change in hardness occurs wheri upTflectpd X-ravs 

 -are used. I accordingly repeated Mr. t»limpton's 



