September 30, 19 15] 



NATURE 



117 



leading dimensions and weights, and such data 

 is then used to show how the performance of an 

 aeroplane may be calculated. The calculation 

 involves the use of aerodynamical data already 

 described, and also means of estimating" the per- 

 formance of propellers. The usual theory of pro- 

 peller design is given, and although this now 

 appears to be in need of improvement, the results 

 obtained by its use have been of very great value. 

 In addition to the calculations relating to equili- 

 brium a chapter is included on the question of 

 stability. 



The book closes with a short description qnd 

 discussion of aero-engines, and the whole appears 

 to be very sound. The impression is left, how- 

 ever, that in the attempt to be concise too little 

 assistance has been given to the reader. Like the 

 book on aero-engines, by the same publisher, the 

 production is excellent, the . illustrations being 

 particularly clear. 



THE VISIBILITY OF DISTANT OBJECTS 



IN WARFARE. 



^r*HE long range of weapons employed in 



-L modern warfare has given importance to 

 the study of the appearance of distant objects. A 

 constant contest is taking place between the 

 observer trying to locate the position and numbers 

 of the enemy, and the observer who endeavours 

 by all possible means to conceal these factors. 



Generally speaking, an object becomes indis- 

 tinguishable when its brightness and colour are 

 identical with its surroundings. For this reason 

 such colours as grey and khaki, which blend well 

 with the surroundings, are preferred for modern 

 uniforms. Yet their effectiveness in this respect 

 depends on the nature of the ground over which 

 troops are moving. Khaki is doubtless difficult 

 to distinguish amidst sandy wastes ; grey or green 

 might be better against grass or foliage. Of all 

 colours, red is the most conspicuous at a dis- 

 tance. Not only is it the colour which presents 

 the most vivid contrast with the ordinary back- 

 ground, but there appear to be certain physio- 

 logical factors which accentuate this impression. 

 For example, it is well known that the central 

 region of the eye (which is mainly used for the 

 observation of distant objects) is highly sensitive 

 to the red end of the spectrum and correspond- 

 ingly insensitive to blue and green. It has also 

 been alleged that, owing to the eye lens not being 

 achromatic, most people find it difficult to focus 

 distant blue and violet light; and that such 

 objects readily merge in the landscape because 

 their outlines are hazy and blurred. Skilful gar- 

 deners in designing a flower-bed arrange the blue 

 and lilac flowers in the foreground when possible, 

 and rely on vivid red and orange blooms for a 

 distant effect. 



I It is known, however, that in a dim light the 

 conditions obtaining in full daylight do not apply. 

 In a feeble illumination the eye becomes more or 

 less colour-blind and is highly insensitive to red, 

 which appears dead black, whereas green and 

 blue objects appear an uncanny grey. A party 

 NO. 2396, VOL. 96] 



of men in grey-green uniforms advancing across 

 a grass field in twilight would therefore be ex- 

 tremely difficult to detect. 



All this suggests that the problem of selecting 

 an inconspicuous uniform is a complicated one, 

 especially when it is borne in mind that it is also 

 important that bodies of men, besides being in- 

 conspicuous to the enemy, should be clearly visible 

 to their own side. It has been suggested that 

 this condition might be secured by differentiation 

 in the colouring of the front and back of the 

 uniform. 



It is interesting to observe that on several 

 occasions during the present war scouts have 

 taken special steps to accommodate the colour of 

 their clothing to the surroundings. For example, 

 it is stated that the Germans provided some of 

 their men with white uniforms in order to match 

 the snow in the Polish campaign, and that the 

 Turkish snipers in Gallipoli painted their hands 

 and faces green so as to be indistinguishable 

 amidst foliage. 



It is clearly more difficult to secure resemblance 

 to surfaces which are constantly changing, such 

 as the sky and the sea. But there is a second 

 principle which can also be used to secure in- 

 conspicuousness in these circumstances, namely, 

 what may be termed the "patchwork" principle. 

 This is based on the experience that the outlines 

 of an object may be rendered difficult to distin- 

 guish by breaking up its surface with stripes and 

 patches. This method has been applied to air- 

 craft and hydroplanes, and to forts and tem- 

 porary defences of various kinds. Besides paint- 

 ing the hulls and funnels of warships a slaty-blue 

 or "battle-grey," experiments have been made 

 with mottled patches of black and irregular ser- 

 pentine black lines painted on a grey background. 

 Some experiments in this direction recently carried 

 out in the United States Navy are said to have 

 been very successful, and it may be only a ques- 

 tion of time before Dreadnoughts are rendered 

 practically invisible at the long range of modern 

 sea-battles. 



A combination of patchwork and imitation of 

 surroundings may also be applied with good 

 results in order to conceal aerodromes and similar 

 objects. For example, if the adjacent ground is 

 cleared and the grass is scraped away at intervals, 

 leaving bare patches, and if the aerodrome itself 

 is painted a patchwork of brown and green, 

 identification from above becomes very difficult. 

 Yet another instance is furnished by the recent 

 correspondence in the Times regarding the colour 

 of sandbags. It is stated that the Germans insert 

 black sandbags at intervals among those of lighter 

 tint. An officer at the front wrote : — " ... It 

 was the first thing I noticed about the German 

 trenches. Their patchwork device made it im- 

 possible to spy their loopholes, whereas ours take 

 a long time to build and then are easily seen." 



A device such as this, which would apparently 

 save a considerable number of lives in a long cam- 

 paign, is well worth attention. 



There remains one other possible device for 

 concealing objects which, although difficult to 



