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NATURE 



[July io, 1919 



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.] 



Military Camouflage. 



Articles on military camouflaj*e have frequently 

 appeared in both scienlitic and popular journals, but 

 the picture drawn is invariably ver\ different from 

 the reality as seen by those who made the camou- 

 flage. The following remarks attempt to correct some 

 of these inaccuracies by setting out as clearly as 

 possible the conditions under which the military 

 camoufleur worked. 



Military information is obtained by horizontal 

 observation from first-line trenches, by horizontal 

 and oblique observation from kite-balloons, and 

 by vertical observation from aeroplanes. Hori- 

 zontal observation has a limited field of a few miles, 

 whereas vertical observation is limited only by the 

 distance an aeroplane can cover and return. The 

 latter is thus -of much the greater importance, 

 especially because with the aid of photography very 

 much greater detail can be obtained than by direct 

 observation. 



Even in forward areas open to horizontal observa- 

 tion camouflage requires to be proof against aerial 

 photographv. What escapes the long-focus camera 

 carrving rapid and fine-grained plates will be passed 

 bv the eve even aided by optical instruments. 



An example will illustrate most of the governing 

 factors. Suppose that it has been decided to conceal 

 a machine-gun emplacement by an artificial haystack. 

 Either it must take the place of an existing stack or 

 recently cut grass must account for its sudden appear- 

 ance in the photograph. The comparison of photo- 

 graphs taken at different dates was a routine practice. 

 The artificial stack must be of the size and form of 

 haystacks of tht> neighbourhood ; any defect in these 

 respects would be recognised photographically by 

 measurement of its cast shadow or by exaggerated 

 stereoscopic examination. .'\ny two photographs taken 

 from about the same height suffice for stereoscopic 

 examination. 



If the stack be made of artificial material, such as 

 canvas, and matched in colour to the eye, it may be dis- 

 covered bv the use of colour-filters or plates sensitive 

 to special regions of the spectrum. The Germans 

 used a verv raoid plate the sensitiveness of which 

 was almost confined to the green region. Toi ensure 

 that the colour composition of the artificial was not 

 at fault, it is best to use hay or straw in its con- 

 struction. This is best for another reason in order 

 to copy so far as oossible the texture of the natural, 

 for under the changing angles of illumination in 

 Nature it is not possible to match constantlv a rough 

 surface bv a smooth artificial, and this difference is 

 likelv to be seen sooner or later bv the reader of aerial 

 photographs. It is clear, therefore, that to defeat 

 the aerial camera an exact copy is required. Amongst 

 animals such concealment is comparable with that of 

 Kallima, the leaf-butterfly. What may be termed 

 impressionistic methods commonly found in animal 

 coloration are quite useless for the defeat of the reader 

 of aerial photoi^raphs. For concealment against hori- 

 zontal observation the same methods had to be em- 

 oloved, as the object is constantlv being scrutinised 

 bv manv eves, aided bv binoculars, colour-filters, etc., 

 exercising svstematic observation, as well as being 

 subject to vertical observation. An observation post 

 in a sandbag parapet had to be concealed by a wire- 

 NO. 2593, VOL. 103] 



gauze screen modelled to resemble exactly the sandbag 

 removed for the purpose. 



Many military objects were successfully concealed 

 by erecting flat covers over them. These were made 

 of fish- or wire-netting threaded with strips of canvas 

 painted green or brown, according as to whether the 

 surroundings were grass or earth. The paints used 

 were made to give as nearly as possible the same 

 colour-composition as grass, earth, etc. The required 

 texture was obtained by leaving dtie ends of the canvas 

 strips long. The cast shadow was concealed by 

 gradually thinning out the canvas strips towards the 

 edge, so that no defined shadow was cast. Under 

 such covers, and more especially on rough ground, 

 guns and other military objects were successfully con- 

 cealed. This is, again,' a case of exact copying except 

 for the cast shadow, which is concealed by thinning 

 out the edge in a manner similar to the standing coats 

 and serrated edges of some animals. 



The difficultv of concealing guns was greatly in- 

 creased because they could be located with consider- 

 able accuracy by flash-spotting and sound-ranging,, 

 though to pin-prick them on the map detection by 

 aerial photography was required. Still, these other 

 methods of detection narrowed down the area to be 

 examined often to the size of a halfpenny. 



Further, the reader of aerial photographs became 

 verv skilled in detecting the presence of concealed 

 objects from signs, tracks, moved earth, unusual agri- 

 cultural activities, activities along roads and rails, and 

 in manv other places distant from the object, so that 

 a camouflaged object was likelv to be subject to the 

 minutest examination and the smallest defect unlikely 

 to be overlooked. 



Meticulous care in the smallest detail was, therefore, 

 essential, especiallv because a detected camouflage 

 gave a false security. The degree in which accuracy 

 was necessarv mav be conveyed by the following 

 example. An" attempt was made to represent a light- 

 railwav track bv dark lines painted upon canvas, the 

 canvas' representing ballast. The painted rail was not 

 so dark in the photograph as the real rail. It was 

 found essential to make a raised rail of rope and 

 canvas in order to cast the same shadow as a steel 

 rail. 



Unfortunatelv, esoeciallv during the early stages of 

 the war, commanding officers often did their own 

 camouflas-Mng. This work almost invariably consisted 

 of imitative' painting, generally of- an exceedingly 

 childish kind," or, on the other hand, they succeeded 

 in making the object more conspicuous by the use of 

 conventional patterns in gaudy colour. In fact, paint- 

 ing for land camouflage was of no value except 

 possiblv to lower the reflection at night from, say, a 

 hospital roof bv the use of dark f?reen or black paint 

 when no texture was available. It is from the work 

 carried out without technical advice from the Camou- 

 flage Corps that the public has been led astray, and 

 it is to counteract this incorrect impression that the 

 foregoing description mav be of value. 



Adrian Kt-eix. 



J. C. MOTTRAM. 



Question Relating to Prime Numbers. 



j The reply to Mr. Mallock's inquiry in Natire of 



' Tune 19 is "that if n denotes the mtii prime number 



(counting from unitv), then the ratio of m to n/log,, »; 



tends to unitv as m tends to infinity. It follows that 



an approximate expression for n is mlog„w. 



These results (known as the prime number theorem) 



were conjectured bv Legendre, and were first proved 



about a Quarter of a centurv ago by Hadamard and 



' bv de la Vallee Poussin. References to the somewhat 



