84 



NATURE 



[April 3, 19 19 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for 

 opinions expressed by his cortrespondents. 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.] 



The Colour of the Scales of Iridescent insects in 

 Transmitted Light. 



Curious as it may seem, the origin of the brilUant 

 metalHc and iridescent colours in birds and insects 

 has never been satisfactorily explained, though it is 

 generally supposed among naturalists and others that 

 they are in some way produced by the interference of 

 light at the surfaces of thin plates, as in the soap- 

 bubble. 



Nevertheless, various other explanations have been 

 put forward, and Michelson, with the weight of his 

 great authority, decides unequivocally that they are 

 due to selective reflection at the surface of a very 

 opaque film, as in the case of metals or dry films of 

 aniline dyes, F"or this conclusion he relies upon the 

 "rigorous optical test of the measurement of the 

 phase-difference and amplitude ratios " when polarised 

 light is reflected. This view has, however, by no 

 means gone unchallenged by Lord Rayleigh, Mallock, 

 and others. 



Whatever the truth of the matter may finally prove 

 to be,_ both those who uphold the theory of selective 

 reflection, and those who uphold the theory of inter- 

 ference, emphasise the fact that the light transmitted 

 through the structures must be complementary to the 

 light which is reflected. 



Thus Michelson says : " In the cases which could 

 be investigated for this relation (unfortunately rather 

 few-) the transmitted light is complementary" to that 

 which is reflected"; and Mallock says: "In cases 

 where the structure is transparent it transmits the 

 complementary colour with nearly the same intensity 

 as the colour reflected." Probably the theorv of selec- 

 tive reflection would require the transmitted colours 

 to be more- vivid than a theorv of "thin plates"; and. 

 as Lord Rayleigh has said, the transmitted colour of 

 the surface layer oi beetles' wings (or Emails chicht of 

 Biedermann which he obtained by maceration in 

 HNO3, etc.) "is not nearly so full as it would be if 

 due to anything like an aniline dve." 



In spite of what is said by Michelson and Mallock, 

 there are a number of iridescent scales in Lepidoptera, 

 Hypolimnas bolina, etc., which, though perfectly 

 transparent, are absolutely colourless. There is the 

 still more anomalous case of the two surface layers 

 of scales of Morpho achilles, and other similar species. 

 I should be greatly interested if any of vour readers 

 could ofi'er a probable explanation." The appearance 

 of these scales is as follows :— Thev are perfectlv 

 transparent, closely striated, verv thin (probably no't 

 much more than 0-5/^). and by reflected light thev are 

 a bright light blue, which is practicallv unaltered by 

 the an^le of^ the incident light. In situ thev show 

 faint dtfi'ractlon colours due to their striation. But 

 the point to which I wish to direct attention is that, 

 when examined by transmitted light under the micro- 

 scope, they still have a blue colour, which is, if any- 

 thing, more saturated than that shown bv reflected 

 light. This colour, though not unlike the " optical 

 blue" of the sea and other fine suspensions, cannot 

 be due to a similar cause, since the light transmitted 

 is not red. It is clearlv visible with a Zeiss 

 A A objective, but practically invisible with higher 

 powers— D D, etc. Moreover, if the scales are 

 examined with an A A objective, and the condenser 

 racked down, the blue colour will gradually disappear 

 NO. 2579, VOL. 103] 



as the condenser is raised until it becomes invisible 

 with critical illumination. These scales have, as a 

 matter of fact, been mentioned by Biedermann, but 

 he entirely fails to notice their significance. 



H. Onslow. 

 3 Selwyn Gardens, Cambridge, March 15. 



Matter and Radiation. 



The theory that matter only radiates energy to 

 matter, as suggested by Dr. Shapley and Prof. Soddy 

 in their interesting letters (March 13 and 20), would 

 certainly solve the great problem of solar energy, and 

 is in many ways attractive." But; is not the evidence 

 against it very strong? Amongst various arguments 

 that may be advanced against it, let us consider that 

 of the difficulty of reconciling it with the existing 

 surface temperature of the earth. 



It is evident that there is an approximate balance 

 between the radiation received by the earth from the 

 sun and that lost, the latter being very slightly larger 

 owing to the heat conducted outwards from the centre. 

 The mean temperature of the earth is about what we 

 would expect on the assumption that it is receiving 

 heat at a known and measured rate from the sun 

 and radiating it uniformly in all directions in accord- 

 ance with Stefan's law. The fraction of the total solid 

 angle which is subtended by matter is apparently 

 almost infinitesimal, yet the quantity of heat radiated 

 agrees with that deduced from experiments in which 

 the radiating body is entirely surrounded with matter. 

 If we assume that surrounding matter only influences 

 the distribution, and not the total flux of radiation, we 

 are led into all sorts of further difficulties. For 

 example, a large part of the radiation from the dark 

 side of Venus would have to be directed towards the 

 earth, so that the radiation received from Venus 

 should be comparable with that received by Venus 

 from the sun. 



Another way of turning the same argument is to 

 consider an isolated solar system. No heat could 

 escape, so it would resemble a system in a perfectly 

 reflecting envelope. Equilibrium would then only be 

 reached when all the members had attained the same 

 temperature. The isolation of the actual solar system 

 in space should certainlv be sufficiently close to ensure 

 that its members would attain a temperature approxi- 

 mating to that of the sun. 



It is possible that this argument is not new, and 

 that there may be some way of evading it, but, as it 

 appears to be difficult to do so, I think it is worthy 

 of consideration. Horace H. Poole. 



Physical Laboratory, Trinity College, 

 Dublin, March 26. 



National Fisheries. 



In Nature of March 13 is published a paragraph 

 on British fisheries in which the following passage 

 occurs: — "To the trade, fish that is scarce and dear 

 is easier to handle than, and at least as profitable as, 

 fish that is cheap and plentiful. From the point of 

 view of the consumer and of the State, cheap food, a 

 large and prosperous fishing population, and, if pos- 

 sible, some revenue, ought to be the objects of recon- 

 struction of the industries concerned." 



The note suggests an antithesis which has no 

 foundation in fact. The fishing industry — producers, 

 research workers, and the "trade" — ^bases the claim 

 for reconstruction, which this association has voiced,, 

 on the fact that it desires to supply the nation with 

 cheap fish. But cheap fish can be supplied only when 

 plentifur catches and regular catches are assured, and 

 to ensure regularity — that is the real crux — without 



