INTRODUCTION TO OPTICS. xciii 



approaching to whiteness ; but one colour they reflect more than the rest: 

 this predominates over the white, and determines the colour of the body. 

 Since, then, bodies of a pale colour in some degree reflect all the rays of 

 light, in passing through the various colours of the spectrum, they will 

 reflect them all with tolerable brilliancy, but will appear most vivid in the 

 ray of their natural colour. The green leaves, on the contrary, are of a 

 dark colour, bearing a stronger resemblance to black than to white : they 

 have, therefore, a greater tendency to absorb than to reflect rays. Blue 

 often appears green by candle-light, because this light is less pure than 

 that of the sun; and when refracted by a prism, the yellow rays predomi- 

 nate ; and as the admixture of blue and yellow forms green, the super- 

 abundance of yellow gives to blue bodies a greenish hue. 



The sun appears red through a fog, owing to the red rays having 

 a greater momentum, which gives them power to traverse so dense an 

 atmosphere. For the same reason the sun generally appears red at 

 rising and setting : as the increased quantity of atmosphere which the 

 oblique rays must traverse, loaded with the mists and vapours which are 

 usually formed at those times, prevents a large proportion of the other 

 rays from reaching us. The colour of the atmosphere, commonly called 

 the sky, is blue ; now since all the rays traverse it in their passage 

 to the earth, it would be natural to infer that it should be white ; but we 

 must not forget that we see none of the rays which pass from the sun 

 to the earth, excepting those which meet our eyes ; and this happens 

 only if we look at the sun, and thus intercept the rays, in which case, 

 you know, it appears white. The atmosphere is a transparent medium, 

 through which the sun's rays pass freely to the earth ; but when reflected 

 back into the atmosphere, their momentum is considerably diminished, 

 and they have not all of them power to traverse it a second time. The 

 momentum of the blue rays is least; these, therefore, are the most im- 

 peded in their return, and are chiefly reflected by the atmosphere ; or it 

 may be that, without any question of momentum, the colour which the 

 particles of air most readily t reflect is blue just as grass reflects the 

 green, or a rose the red rays. This reflection is performed in every pos- 

 sible direction ; so that wherever we look at the atmosphere, some of 

 these rays fall upon our eyes : hence we see the air of a blue colour. If 

 the atmosphere did not reflect any rays, though the objects on the sur- 

 face of the earth would be illumined, the skies would appear perfectly 

 black. This would not only be very melancholy, but it would be per- 

 nicious to the sight, to be constantly viewing bright objects against a 

 black sky. 



When bodies change their colour, as leaves which wither in autumn, 

 or a spot of ink which produces an iron-mould on linen, it arises from 

 some chemical change, which takes place in the internal arrangement of 

 the parts, by which they lose their tendency to reflect certain colours, 

 and acquire the power of reflecting others. A withered leaf thus no 

 longer reflects the blue rays : it appears, therefore, yellow, or has a slight 

 tendency to reflect several rays which produce a dingy brown colour. 

 An ink-spot on linen at first absorbs all the rays ; but, exposed to the air, 

 it undergoes a chemical change, and the spot partially regains its ten- 

 dency to reflect colours, but with a preference to reflect the yellow 

 rays ; and such is the colour of the iron-mould. 



