Balfour Stewart 153 



coloured flames, and noted the yellow colour imparted to a 

 flame by soda. His observations were published in a book 

 bearing the title " Physical and Literary Essays." Exactly 

 fifty years later, William Hyde Wollaston, who has already 

 been mentioned as the discoverer of palladium and rhodium, 

 examined the blue light at the base of a candle flame through 

 a prism, and described the bright bands which appear in its 

 spectrum. Young repeated the experiments, and committed 

 what is perhaps the one great error of his scientific work, 

 when he ascribed the colours seen to effects of diffraction. 

 In these and most of the subsequent observations, the light 

 to be examined is passed through a slit, and traversing a 

 prism is separated into its components. The eye focussing 

 on the slit, with or without lenses, sees it illuminated by 

 the various elementary vibrations which the original light 

 may emit. These vibrations show themselves, therefore, as 

 luminous lines, which are images of the slit. The whole 

 appearance is called a spectrum, of which it is customary to 

 speak as consisting of " lines," a misleading term, because it 

 implies that the " line " is a characteristic of the substance, 

 while it is only an incident of the instrument by which the 

 spectrum is examined. The expression, having been univer- 

 sally adopted, may be retained with the understanding that 

 it is the position of the line which indicates the nature of the 

 light vibration, and therefore characterizes the luminous body. 

 Sir John Herschel investigated coloured flames in 1823, and 

 made two significant observations : " The colours thus 

 communicated by the different gases to flame afford, in 

 many cases, a ready and neat way of detecting extremely 

 minute quantities of them," and " no doubt these tints 

 arise from the molecules of the colouring matter reduced to 

 vapour, and held in a state of violent motion." Fox Talbot 

 in 1826 looked at the red lights occasionally used to illuminate 

 the stage in theatres. He correctly ascribed a red line to 

 nitre, but believed the yellow sodium line to be due to sulphur 

 or water. Eight years later Talbot returned to the subject, 

 and clearly pointed out that " optical analysis can distinguish 

 the minutest portions of these substances (lithium and 

 strontium) from each other with as much certainty, if not 

 more, than any other known method." He also offered the 



