Among the wonders of science there are perhaps none more surprising than the effects produced upon coloured objects by illuminating them with homogeneous light, or light of one colour. The light which emanates from the sun, and by which all the objects of the material world are exhibited to us, is composed of three different colours, _red_, _yellow_, and _blue_, by the mixture of which in different proportions all the various hues of nature may be produced. These three colours, when mixed in the proportion in which they occur in the sun’s rays, compose a purely white light; but if any body on which this white light falls shall absorb, or stop, or detain within its substance any part of any one or more of these simple colours, it will appear to the eye of that colour which arises from the mixture of all the rays which it does not absorb, or of that colour which white light would have if deprived of the colours which are absorbed. Scarlet cloth, for example, absorbs most of the blue rays and many of the yellow, and hence appears _red_. Yellow cloth absorbs most of the blue and many of the red rays, and therefore appears yellow; and blue cloth absorbs most of the yellow and red rays. If we were to illuminate the _scarlet_ cloth with pure and unmixed _yellow_ light, it would appear _yellow_, because the scarlet cloth does not absorb all the yellow rays, but reflects some of them; and if we illuminate _blue_ cloth with yellow light, it will appear nearly _black_, because it absorbs all the yellow light, and reflects almost none of it. But whatever be the nature and colour of the bodies on which the yellow light falls, the light which it reflects must be yellow, for no other light falls upon them, and those which are not capable of reflecting yellow light must appear absolutely black, however brilliant be their colour in the light of day. [Illustration: _Fig. 21._] As the methods now discovered of producing yellow light in abundance were not known to the ancient conjurors, nor even to those of later times, they have never availed themselves of this valuable resource. It has been long known that salt thrown into the wick of a flame produces yellow light, but this light is mixed with blue and green rays, and is, besides, so small in quantity, that it illuminates objects only that are in the immediate vicinity of the flame. A method which I have found capable of producing it in abundance is shown in Fig. 21, where AB is a lamp, containing at A a large quantity of alcohol and water, or ardent spirits, which gradually descends into a platina or metallic cup D. This cup is strongly heated by a spirit-lamp L, inclosed in a dark lantern, and when the diluted alcohol in D is inflamed, it will burn with a fierce and powerful yellow flame. If the flame should not be perfectly yellow, owing to an excess of alcohol, a proportion of salt thrown into the cup will answer the same purpose as a further dilution of the alcohol.[12]