PDF - Wallace Online
PDF - Wallace Online PDF - Wallace Online
356 TROPICAL NATUREexternal objects, and which form one of the great charms ofour existence. Primary colours would therefore be asnumerous as the different wave-lengths of the visible radiations,if we could appreciateall their differences; whilesecondary or compound colours, caused by the simultaneousaction of any combination of rays of different wave-lengths,must be still more numerous.In order to account for the fact that all colours appear tous to be produced by combinations of three primary coloursred, green, and violet it is believed that we have threesets of nerve fibres in the retina, each of which is capable ofbeing excited by all rays, but that one set is excited most bythe larger or red waves, another by the medium or greenwaves, and the third set chiefly by the violet or smallestwaves of light ;and when all three sets are excited togetherin proper proportions we see white. This view is supportedby the phenomena of colour-blindness, which are explicable onthe theory that one of these sets of nerve-fibres (usually thatadapted to perceive red) has lost its sensibility, causing allcolours to appear as if the red rays were abstracted fromthem.It is a property of these various radiations that they areunequally refracted or bent in passing obliquely throughtransparent bodies, the longer waves being least refracted, theshorter most. Hence it becomes possible to analyse white orany other light into its component rays. A small ray ofsunlight, for example, which would produce a white spoton a wall, if passed through a prism, is lengthened out into aband of coloured light, exactly corresponding to the colours ofthe rainbow. Any one colour can thus be isolated andseparately examined ;and by means of reflecting mirrors theseparate colours can be again compounded in various ways,and the resulting colours observed. This band of colouredlight is called a spectrum, and the instrument by which thespectra of various kinds of light are examined is called aspectroscope. This branch of the subject has, however, nodirect bearing on the mode in which the colours of livingthings are produced, and it has only been alluded to in orderto complete our sketch of the nature of colour.The colours which we perceive in material substances are
v COLOURS OF ANIMALS 857produced either by the absorption or by the interference ofsome of the rays which form white light. Pigmental orabsorption-colours are the most frequent, comprising all theopaque tints of flowers and insects, and all the colours of dyesand pigments. They are caused by rays of certain wavelengthsbeing absorbed, while the remaining rays are reflectedand give rise to the sensation of colour. When all the colourproducingrays are reflected in due proportion, the colour ofthe object is white ;when all are absorbed the colour is black.If blue rays only are absorbed the resulting colour is orangered;and generally, whatever colour an object appears to us,it is because the complementary colours are absorbed by it.The reason why rays of only certain refrangibilities are reflected,and the rest of the incident light absorbed by eachsubstance, is supposed to depend upon the molecular structureof the body. Chemical action almost always implies changeof molecular structure; hence chemical action is the mostpotent cause of change of colour. Sometimes simple solutionin water effects a marvellous change, as in the case of thewell-known aniline dyes; the magenta and violet dyesexhibiting, when in the solid form, various shades of goldenor bronzy metallic green.Heat alone often produces change of colour without effectingany chemical change. Mr. Ackroyd has investigated thissubject, 1 and has shown that a large number of bodies arechanged by heat, returning to their normal colour whencooled, and that this change is almost always in the directionof the less refrangible rays or longer wave-lengths; and heconnects the change with the molecular expansion caused byheat. As examples may be mentioned mercuric oxide, whichis orange yellow, but which changes to orange, red, and brownwhen heated ;chromic oxide, which is green, and changes toyellow ; cinnabar, which is scarlet, and changes to puce and;metaborate of copper, which is blue, and changes to greenand greenish yellow.How Animal Colours are ProducedThe colouring matters of animals are very varied. Copperhas been found in the red pigment of the wing of the turaco,1 " Metachromatism, or Colour-Change," Chemical News, August 1876.
- Page 322 and 323: TROPICAL NATUREMAMMALIAMonkeysThe h
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- Page 344 and 345: TROPICAL NATUREhalf completed.In th
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- Page 360 and 361: 344 TROPICAL NATUREprotecting them
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- Page 370 and 371: 354 TROPICAL NATUREMr. Darwin's Des
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- Page 392 and 393: 376 TROPICAL NATUREtion, and what m
- Page 394 and 395: 378 TROPICAL NATURENatural Selectio
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- Page 412 and 413: 396 TROPICAL NATUREthese differ gre
- Page 414 and 415: 398 TROPICAL NATUREvery minute and
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v COLOURS OF ANIMALS 857produced either by the absorption or by the interference ofsome of the rays which form white light. Pigmental orabsorption-colours are the most frequent, comprising all theopaque tints of flowers and insects, and all the colours of dyesand pigments. They are caused by rays of certain wavelengthsbeing absorbed, while the remaining rays are reflectedand give rise to the sensation of colour. When all the colourproducingrays are reflected in due proportion, the colour ofthe object is white ;when all are absorbed the colour is black.If blue rays only are absorbed the resulting colour is orangered;and generally, whatever colour an object appears to us,it is because the complementary colours are absorbed by it.The reason why rays of only certain refrangibilities are reflected,and the rest of the incident light absorbed by eachsubstance, is supposed to depend upon the molecular structureof the body. Chemical action almost always implies changeof molecular structure; hence chemical action is the mostpotent cause of change of colour. Sometimes simple solutionin water effects a marvellous change, as in the case of thewell-known aniline dyes; the magenta and violet dyesexhibiting, when in the solid form, various shades of goldenor bronzy metallic green.Heat alone often produces change of colour without effectingany chemical change. Mr. Ackroyd has investigated thissubject, 1 and has shown that a large number of bodies arechanged by heat, returning to their normal colour whencooled, and that this change is almost always in the directionof the less refrangible rays or longer wave-lengths; and heconnects the change with the molecular expansion caused byheat. As examples may be mentioned mercuric oxide, whichis orange yellow, but which changes to orange, red, and brownwhen heated ;chromic oxide, which is green, and changes toyellow ; cinnabar, which is scarlet, and changes to puce and;metaborate of copper, which is blue, and changes to greenand greenish yellow.How Animal Colours are ProducedThe colouring matters of animals are very varied. Copperhas been found in the red pigment of the wing of the turaco,1 " Metachromatism, or Colour-Change," Chemical News, August 1876.
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