Memoirs on the Absorption of Light. 345 



ties and peculiarities, of which he had not the least conception, have been 

 found to belong to the several parts of which the spectrum consists, and 

 the media capable of transmitting light. 



The first great discovery, in this line, since Newton's time, was the dif- 

 ference of the dispersive powers, by which different media, under equal 

 angles of refraction, separate ihe extreme rays unequally. Hence the pos- 

 sibility of that grand improvement in practical optics, the achromatic tele- 

 scope. But this is far from a perfect instrument, and, if we carry the 

 magnifying powers of our telescopes beyond a certain extent, we speedily 

 become sensible of the limit which the nature of the media, of which its 

 lenses consist, places to our further progress. In microscopes, however, 

 achromatic lenses had rarely, if ever, been adopted, and in these, accord- 

 ingly, the original imperfection of refracting telescopes existed in all its 

 force. To obviate the inconvenience of chromatic aberration in these in- 

 struments, two methods occurred to Dr Brewster, viz. Either to extinguish 

 all the rays, but those of one colour, by the use of coloured glasses, and 

 thus to destroy the erratic light before its entry into the eye; or, 2dly, To 

 use homogeneous light ah initio for the illumination of the object viewed. 

 The former of these methods, however, is either imperfect, or is attended 

 with the loss of so much light, as to render vision obscure, and, though 

 affording a certain degree of advantage, Dr Brewster was induced to aban- 

 don this method for the above reasons. 



The discovery of a source of homogeneous light next occupied his atten- 

 tion, and, after numerous trials, he ascertained the remarkable fact, that 

 almost all bodies in which the combustion was imperfect, such as paper, 

 linen, cotton, &c. gave a light in which the homogeneous yellow rays pre- 

 dominated — that the yellow light increased with the humidity of these 

 bodies — and that a great proportion of the same light was generated, when 

 various flames were urged mechanically with a blow-pipe or pair of bellows. 

 He thence concludes, that the yellow rays appear to be the produce of an 

 imperfect combustion. We may observe, however, that combustion is not 

 necessarily imperfect in the flames above enumerated. Most inflammable 

 bodies, moreover, in a state of incipient or weak combustion, discharge, 

 not a yellow, but a blue flame. The blue flame, at the bottom of a 

 candle, is known to every one ; that of burning sulphur (in its usual weak 

 state of combustion) is hardly less familiar ; and the deep blue flame of a 

 piece of paper, scorched on the under side till the upper takes fire, is mat- 

 ter of wonder and delight to every child who plays with fire. It is only 

 when the combustion becomes full and violent that the yellow light be- 

 gins to predominate in flames. Mr Herschel observes, that sulphur, urged 

 to the utmost intensity of combustion, by projecting it into a white-hot 

 crucible, discharges a homogeneous yellow light, but as the intensity of the 

 heat diminishes, the blue and green spectra appear. The flame of oil urged 

 by bellows, (in which state its combustion is undoubtedly most complete,) 

 has been observed by Mr Fraunhofer, (in a conversation with ourselves,) 

 as well as by Dr Brewster, to consist principally or wholly of yellow 



