714 



NATURE 



[August 4, 192 1 



Letters to the Editor. 



YThe 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.] 



The "Flight" of Flying-fish. 



Having read with interest the letters by Prof. 

 Wood-Jones and Mr. Julian S. Huxley in Nature 

 for April 21 and 28 respectively, I send some 

 observations of my own which seem to have a 

 bearing on this subject. In the early 'nineties I was 

 engaged in the development of the meteorological kite 

 of the Hargrave pattern, which was adopted at the 

 Blue Hill Observatory for lifting self-recording ap- 

 paratus in the air, and later adopted by the various 

 bureaus of the world for aerological research. This 

 work brought me in contact with the early pioneer 

 workers on the problem of flight in the United States — 

 Langley, Chanute, the Wrights, Cabbot, Means, Millet, 

 and others — and I occasionally co-operated in experi- 

 ments on the lifting and driving powers of various 

 devices. One of these was a device in which a stiflf rod 

 had attached to one end a flexible rod of bamboo, one 

 end of the bamboo strip being tied near the end of the 

 rigid rod and the other about one-fourth of the way 

 down, so that the bamboo rod formed a loop, over 

 which was drawn a covering of cloth. Now, if one 

 took the free end of the rigid rod and waved the end 

 containing the bamboo loop up and down, he was 

 immediately turned round by a forward motion of 

 the outer end of the rigid rod. The reason of this 

 clearly was that when he lifted the rigid rod upward 

 the flexible loop bent downward, and there was a 

 component of air pressure forward, while when he 

 moved the rigid rod downward the flexible loop bent 

 upward, and there was still a component of air pres- 

 sure forward. When vibrating the rod up and down 

 there was a persistent forward thrust, and this thrust 

 was so great when the vibration was rapid that the 

 operator was turned completely around in his tracks 

 as on a pivot. 



In 1905 I was in charge of the Tieserenc de Bort 

 and Rotch Expedition for exploring the atmosphere 

 with balloons and kites over the tropical part of the 

 Atlantic. The Otaria, on which we travelled, was a 

 small boat not much more than 100 ft. in length, 

 with the decks near the water, so that I had an 

 excellent opportunity of studying the movements of 

 flying-fish, which we saw in great numbers. 



As these fish left the water the powerful lateral 

 strokes of the tail which lifted them into the air could 

 be plainly seen. As they rose into the air the pectoral 

 fins vibrated with great rapidity, and my earlier ex- 

 periments with the rod and flexible web led me to 

 believe that exactly this same principle was used by 

 the flying-fish to drive itself forward. The forward 

 part of the pectoral fin is rigid and the rear flexible, 

 so that its rapid vibration gives a strong propelling 

 force. When the fish had gained velocity and the 

 rising impulse given by the tail had culminated, the 

 fins ceased to vibrate and were used as aeroplanes, 

 on which it glided forward, slowly sinking until its 

 tail touched the water, when another lateral stroke 

 lifted it into the air and the process was repeated. 

 In this way the fish could remain in the air for long 

 flights when necessary. The only way in which the 

 motion differs from the flight of birds is that the 

 vibration of the fins probably gives no lifting force, 

 but only a forward driving force, and the fish needs 

 to depend on the tail-strokes for the lift. Had the 



NO. 2701, VOL. 107] 



fish developed a concave under-surface of the fin it 

 could probably have obtained both lifting and pro- 

 pelling force from the fins. H. H. Clayton. 

 Oficina Meteorologica, Buenos Aires, 

 June 20. 



The Colours of Breathed-on Plates. 



The phenomena of breath-figures on glass are of 

 considerable interest, and have been written upon in 

 the columns of Nature by the late Lord Rayleigh, 

 Dr. John Aitken, and others. One specially interest- 

 ing aspect of the subject to which I have recently 

 devoted some attention is the explanation of the 

 beautiful optical effects exhibited by breathed-on plates 

 of glass. If a clean, cold plate of glass is lightly 

 breathed on and then held in front of the eye, and if 

 a small distant source of light is viewed through it, 

 coloured haloes will be seen surrounding the source. 

 The characteristic feature of the halo exhibited by a 

 moderately heavy (but not too heavy) deposit is that 

 the outermost ring in it is achromatic, with a reddish 

 or brown inner margin, followed inside by a succes- 

 sion of rings of various colours. As the film of 

 moisture evaporates, the halo contracts and the coloured 

 rings move inwards, ultimately disappearing at the 

 centre of the halo. The entire halo presents a 

 radiating fibrous structure. 



The explanation of these phenomena presents some 

 difficulties. One is tempted to suppose (as, indeed, 

 Donl^ and Exner have already) either that the 

 minute droplets of water condensed on the plate 

 which diffract the light are of approximately equal 

 size or that they are arranged at more or less 

 constant distances from each other. A microscopic 

 examination of the condensed film shows, how- 

 ever, that neither of these suppositions is anywhere 

 near the truth. The size of the individual droplets 

 shows a variation of several hundred per cent., and 

 their arrangement in the film is entirely irregular, 

 being determined presumably by the presence of in- 

 visible condensation nuclei on the surface of the plate 

 — a view that is strongly supported by the fact that 

 successive deposits on the plate are seen under the 

 microscope to pi-eserve the same configuration with a 

 surprising degree of accuracy. Further, if the size of 

 the droplets were the determining factor in the pro- 

 duction of the diffraction haloes, it would be difficult 

 to understand why as they evaporate the rings in the 

 halo should contract in size. 



These facts necessitate an entirely different sup- 

 position regarding the element of regularity in the 

 film which enables it to give rise to a recognisable 

 system of coloured diffraction haloes. Measurements 

 I have made seem to show that the droplets in the 

 film — whether large or small — have practically all the 

 same angle of contact with the surface of the plate, 

 this angle of contact diminishing as the film 

 evaporates. The formation of the coloured haloes is, 

 on this view, due to the passage of the light through 

 the minute lens-shaped droplets ; the maximum devia- 

 tion of the light determined by the common angle of 

 contact fixes the position of the outermost achromatic 

 halo, and the colour-sequence following within It 

 would be practically the same for all the droplets 

 irrespective of their size. This would also furnish a 

 satisifactory explanation of the contraction of the halo 

 as the film evaporates. C. V. Raman. 



22 Oxford Road, Putney, S.W.15, July 26. 



Mutations and Evolution. 



The article on my recent little book on " Mutations 

 and Evolution " in Nature of July 14, p. 636, shows 

 such insight in the exposition of some of the views 



