COLOUR. 



81 



plant has naturally determinate colours, by which it 

 may be recognised ; and in mineralogy, each species 

 of mineral presents a particular colour or suite of 

 colours, by which it may also be identified. In 

 meteorology, the aerial tints, the meteors, and the 

 atmospherical phenomena peculiar to certain coun- 

 tries, can only be appreciated by a correct perception 

 of the different colours which characterise their ap- 

 pearance ; while in hydrography, a similar power of 

 discrimination is necessary, to describe the varieties 

 of colour which occur in the seas, lakes, and rivers, 

 which adorn while they fertilise the surface of the 

 earth. Hence every naturalist should be well ac- 

 quainted with the philosophy of colours; he should 

 be able to discriminate them accurately ; he should 

 know the effects which their several combinations 

 will produce ; and should understand the circum- 

 stances by which they may be modified or entirely 

 changed. 



The colour of all natural bodies, whether we con- 

 template the gorgeous hues of sunrise or sunset, the 

 iridescent plumage of a bird, or the tints that varie- 

 gate the petals of a flower, must be derived either 

 from the light of the sun, or else be inherent, like 

 any other property of matter, in the substance of the 

 bodies themselves, the light in that case being only 

 the passive medium through which their effect is 

 communicated. The majority of philosophers concur 

 in ascribing their origin to some change in the con- 

 stitution or condition of light, yet their opinion 

 concerning the manner in which this change takes 

 place, must depend entirely on the views they enter- 

 tain concerning the nature and properties of light. 

 On this subject two theories are at present very ably 

 maintained by their respective advocates ; the one 

 termed the Newtonian or molecular; the other 

 the undulatory theory of light; each of which is 

 signally ingenious and beautiful in detail, ancP alike 

 supported by men of the profoundest mathematical 

 skill. According to the Newtonian or molecular 

 doctrine, light is presumed to consist of an infinite 

 number of very minute particles of matter, which 

 proceed in a continuous stream from the sun, or any 

 other luminous body, and which, impinging on the 

 nervous filaments of the retina, with such rapidity, as 

 to allow of no sensible intermission of the impression 

 they excite, induce the sense of vision. On the other 

 hand, according to the undnlatory hypothesis, light 

 is supposed to consist simply in the undulations of 

 an exquisitely subtile and elastic medium or ether, 

 which pervades the whole universe. The waves of 

 this medium, according to the impulse communicated 

 to them by the action of what are called luminous 

 bodies, reach the nerves of the eye and excite the 

 sensation of sight, in the same manner as the sensation 

 of sound is excited in the nerves of the ear by the vibra- 

 tions of air. In the history of science, never perhaps 

 were two theories proposed to explain the same pheno- 

 mena, in which the evidence in favour of each appears 

 to be so fairly balanced ; yet to understand how either 

 may be applied to explain the origin of colours, we 

 must revert to certain properties of light which are 

 immediately connected with this interesting investi- 

 gation. 



Whether the molecular or undulatory theory be 

 adopted, light is observed to move in straight fines, 

 until intercepted in its progress by passing from a 

 medium of one density into a medium of another 

 density; or until interrupted by impinging against 



NAT. HIST. VOL. II. 



some semi-transparent or opaque body. In passing 1 

 from air into water, the rays of light on entering the 

 fluid are observed to deviate from their previous rec- 

 tilinear direction ; they appear to become bent, and 

 this deviation from the straight line is called the 

 refraction of light. Hence when an oar is immersed 

 in water, it appears distorted on account of the rays 

 of light by which it is seen being refracted or bent, 

 on emerging from the water. The extent, however, 

 to which the light deviates from the rectilinear direc- 

 tion varies in different media ; thus, as the atmosphere 

 which surrounds the earth, is denser in its lower 

 than in its upper regions, the light of the sun, in 

 passing from the rarer into the denser strata, becomes 

 more and more bent or refracted ; so that it descends 

 to us in a vertical curved line, for which reason the 

 moon and stars appear to us at a greater altitude 

 than they really are; for which, in astronomical and 

 trigonometrical observations, allowance is always 

 made. The rays of light, however, are not refracted 

 if they fall perpendicularly on the denser medium ; 

 hence the refraction becomes less as the sun ap- 

 proaches to the zenith, and increases as it descends 

 towards the horizon ; accordingly, in the blaze of 

 noon the sun's rays are reflected with silvery and 

 dazzling whiteness from the surface of the waters, 

 w-hereas towards evening, as the sun gradually sets 

 its rays, falling obliquely upon the waters, the waves 

 become illuminated with all the beautiful prismatic 

 colours of the refracted light. Again, when light 

 impinges upon any solid body, it is either almost 

 entirely transmitted through it, in which case the 

 body is transparent, or it is reflected or driven back 

 at a certain angle, just as a marble after striking 

 against a wall is reflected, or driven off again ; and this 

 is what is termed the reflection of light. The surface 

 of metals which are highly polished, or glass quick- 

 silvered at the back to prevent the transmission of 

 the rays, are the best reflectors ; as are also all white 

 surfaces. When the earth is covered with snow, 

 and all objects are shrouded in the same wintry gar- 

 ment, the snowy crystals reflect the sun's light so 

 vividly in all directions, as to excite acute pain iu 

 the eye, an inconvenience much complained of by 

 navigators, who have endeavoured to explore the 

 dreary solitudes of the Polar Regions. Even the 

 Esquimaux, and other natives of those snowy wilder- 

 nesses, are obliged to protect their eyes from this 

 reflection of the solar rays, by a sort of wooden shade 

 which is commonly worn among them. To these two 

 properties refraction and reflection we owe the 

 uniform diffusion of light through the atmosphere ; 

 indeed, had the atmosphere, instead of possessing 

 regions of different density, consisted of an unlimited 

 homogeneous medium, the sun, the moon, and the 

 fixed stars, would have appeared to shine in a firma- 

 ment of darkness ; so exquisitely adapted is the air 

 we breathe, and the sunbeam which illumines our 

 path, to the general economy and beauty of the 

 universe. There is yet, however, connected with 

 air, water, and other media through which light may 

 be transmitted, a curious property to be noticed ; 

 viz. when they are sufficiently dense, they absorb a 

 great quantity of the light which enters into them ; 

 in other words, the light which falls upon such trans- 

 parent media, is greater than the quantity which is 

 either refracted or reflected, so that a portion in some 

 way or other becomes suppressed or lost. For this 

 reason, owing to the absorption of the refracted light, 

 F 



