482 HISTORY OF SCIENCE. 



Fox-Talbot, whose name will always be remembered in connection 

 with the beautiful art of photography, in a paper published in 1826 

 records the results of prismatic examinations of flames, and proposes 

 the use of the prism in detecting the presence of certain substances. 

 " The flame of sulphur and nitre," he says, " contains a red ray of a 

 definite refrangibility, and apparently characteristic of potash salts, as 

 the yellow ray is of soda salts. The red light, however, is not visible 

 in the flame viewed by the naked eye, on account of feeble illumi- 

 nating power, but is detected by the prism." He suggests this prin- 

 ciple : whenever the prism shows a homogeneous ray of any colour to 

 exist in a flame, the ray indicates the presence of a definite chemical 

 compound. Then, after describing the spectrum of a flame coloured 

 by strontia, he makes the observation (which has since been com- 

 pletely realized) : " If this opinion should be correct, a glance at the 

 prismatic spectrum of a flame may show it to contain substances which 

 it would otherwise require a laborious chemical analysis to detect." 

 Some years afterwards (1834), speaking of the spectra of lithium and 

 of strontium, which impart to flames intense red colours not to be 

 distinguished from one another by the unaided eye, he is more posi- 

 tive : " Hence I hesitate not to say that optical analysis can distin- 

 guish the minutest portions of these substances from each other with 

 as much certainty as any other known method, if not with more." 



By this we find that the principle of spectral analysis was clearly 

 enunciated as regards certain substances. The extreme minuteness of 

 the quantity of sodium compounds which suffices to give the intense 

 yellow line was unknown to these early observers, and the constant 

 intrusion of the sodium line in cases where no sodium was thought to 

 be present, was a source of much perplexity. Fox-Talbot was inclined 

 to attribute this line to the presence of water, which he supposed to 

 be the only substance that could possibly be present in the various 

 salts, all of which yielded spectra containing the yellow ray, whatever 

 other lines were also produced. But he found the yellow line showing 

 itself in the spectrum of burning sulphur, which could not be supposed 

 to contain water. We now know that sodium compounds are most 

 widely diffused, and that a trace of them, which would be beyond the 

 ordinary means of detection, even if increased a thousandfold, suffices 

 for the production of the yellow ray. 



Wollaston, as we have already seen, observed spectra in which lines 

 of three classes were represented : he saw first the dark lines in the 

 sun's light, which are often called after Fraunhofer ; second, the bright 

 lines produced by a coloured flame ; and third, the bright lines of the 

 electric spark. All these classes of lines have since been the sub- 

 jects of much research, and the germ of some of the most important 

 discoveries which have been made in spectroscopy was contained in 

 an observation of Sir David Brewster's, by which a fourth class of lines 

 was brought to notice in 1832. Brewster found that the brownish 



