COLOUR. 



83 



definite points or rays, to each of which a different 

 refrangibility belongs, their number must be considered 

 as infinite. The general view now given of the con- 

 stitution of light, according to the molecular theory, 

 will enable us to understand the explanation it assigns 

 for the different colours observed in nature. 



The principle on which all bodies are supposed to 

 derive their colour is briefly this, that the particles of 

 which they are composed" absorb some, and reflect 

 other of the primary and different coloured rays 

 which enter into the composition of white light. 

 The red petal of a rose is presumed to absorb the six 

 other coloured rays, and reflect only the red ray; 

 the blue flower of the harebell to absorb the six other 

 and reflect only the blue ray ; and so on with all 

 objects that reflect the different primary rays disco- 

 verable in the solar spectrum. Those bodies which 

 reflect several of such rays appear of various colours ; 

 those which reflect them all appear white, while those 

 which absorb them all appear black ; and the absorp- 

 tion and reflection of the different coloured rays 

 depends, according to the hypothesis of Sir Isaac 

 Newton, on the magnitude of the particles of which 

 the body is composed. Thus the azure colour of the 

 sky is caused by the particles being very small ; 

 the green colour of vegetables depends on their being 

 somewhat larger ; " the green of vegetables," says 

 Pemberton, one of the most perspicuous of Newton's 

 commentators, " our author judges to be of the third 

 order, partly from the changes they suffer when they 

 wither, turning at first into a greenish or more perfect 

 yellow, and afterwards some of them into an orange 

 or red, which changes seem to be effected by their 

 tinging particles growing denser by the exhalation of 

 their moisture." The change of colour, therefore, 

 which any plant, mineral, or other substance, may 

 undergo, either from the action of light, heat,, or vital 

 or chemical changes, is presumed to depend on some 

 alteration which takes place in the size of its compo- 

 nent particles, which causes other of the primary rays 

 to be reflected. Instead, however, of the mere size of 

 the particles determining the inflection of one or more 

 of the coloured rays, Sir David Brewster supposes that 

 the particles of bodies, solid, fluid, or aeriform, have 

 a specific attraction for the particles of light ; if they 

 exercise a simultaneous and proportional action over 

 all the simple rays, then the body is colourless or 

 transparent ; if they exercise a specific attraction only 

 over some of the simple rays, then the body exhibits 

 a determinate colour from the reflection of the other 

 rays, whether blue, yellow, or red. The colours pro- 

 duced by the refraction and reflection of these primary 

 rays are especially observable in the rainbow, in solar 

 or lunar halos, in the glaciers that, crown the summits 

 of the mountains in Switzerland, and at sunrise or 

 sunset upon the surface of the ocean. Hence too 

 the colour of the sky, the blue and violet rays being 

 most refrangible are more copiously distributed 

 through the atmosphere than the other rays, while 

 the red rays being the least refrangible are not so 

 frequently transmitted ; on account, indeed, of their 

 diminished refrangibility, the most favourable con- 

 dition of the atmosphere for their refraction is obvi- 

 ously when the least vapour or moisture is suspended 

 in it, and accordingly it is a popular observation that 

 a red sky at sunset is a sign of fine weather the next 

 day. In accordance too with what has been noticed 

 concerning the disposition of certain media to absorb 

 light, the air or vapours suspended in it are observed 



often to absorb the red rays ; hence the fiery aspect 

 of the sky at sunrise or sunset, and the flame-coloured 

 masses of clouds which are often seen dispersed even 

 to the zenith. 



But this is not all, for, independent of the colours 

 of natural bodies thus arising Irom the absorption and 

 reflection of the primary rays, Sir Isaac Newton 

 referred the colours of the feathers of birds, the wings 

 of insects, the scales of fishes, &c., to the same cause 

 as the colours exhibited by thin plates. It was 

 ascertained by Boyle, and afterwards by Dr. Hooke, 

 that when substances which appear to be opaque are 

 reduced into very thin layers they display the most 

 beautiful colours. This may be familiarly exemplified 

 by the bubbles of the essential oils, spirit of wine, tur- 

 pentine, or soap and water, which, as they become 

 gradually thin, exhibit the most varied and brilliant 

 tints. So also is it with solid substances. Glass 

 may be blown so thin, or mica split into such fine 

 films, as to display the same lively colours ; and on 

 the same principle, the delicate filaments of a feather, 

 the exquisitely fine membrane of an insect's wing, or 

 very thin scales, exhibit similar radiant hues. 



The colours, however, of the plates vary according 

 as the rays of light fall upon them ; when viewed 

 obliquely, they are not the same as when viewed 

 direct, for which reason the colours in the peacock's 

 tail change as the bird changes the posture of its 

 tail in respect to sight. Hence also the delicate 

 plumage of the little humming bird displays the 

 most exquisitely varied rainbow hues, so that when 

 numbers of them are seen together flitting about a 

 flower, or wantonly describing eccentric gyrations in 

 the sunny air, the perpetual shifting of the colours, as 

 the position of the bird varies, has a most dazzling 

 and brilliant effect. The beautiful colours in the 

 interior of shells are referrible to the same cause, 

 as are also the colours of mother-of-pearl, which, 

 when examined under a powerful microscope, presents 

 a surface covered with minute stria? and parallel 

 waving lines. The explanation of this phenomenon, 

 given by Sir Isaac Newton, was this, that a portion 

 of light is reflected from the upper surface of the 

 thin plate, whether of glass or any other substance ; 

 and that the coloured rays are refracted from the 

 adjacent under surface. According, however, to 

 the modern theory, which ascribes the effect to the 

 doctrine of interference, the light reflected from the 

 second surface of the plate interferes with the light 

 reflected from the first, and as these two pencils of 

 light come from different points of space, they must 

 reach the eye with different lengths of paths ; and the 

 resulting tint arises from the combination of the pencils. 



The Newtonian molecular theory of light, therefore, 

 refers the origin of colours, first, to the decomposition 

 of white light, and the absorption of some, and refrac- 

 tion and reflection of one or more of the primary or 

 different coloured simple rays ; second, to the inter- 

 ference of light by very thin plates or fringes, the light 

 in both cases consisting of subtle material particles 

 emitted by luminous bodies. 



According to the undulatory theory of light, a very 

 different explanation is given concerning the origin 

 and nature of colours ; yet before stating the doctrine 

 it inculcates, we may observe that all the properties 

 and phenomena of light above referred to, its refrac- 

 tion, reflection, and absorption, are upon the undu- 

 latory hypothesis satisfactorily explained. As the 

 undulations of the ether are propagated in a rectilinear 

 F2 



