212 PROGRESS OF SCIENCE IN THE CENTURY. 



furnishes a means of showing that certain kinds of 

 glowing matter known to our terrestrial experience 

 also occur in sun and stars. But the recognition 

 of the importance of this new organon came about 

 very gradually. 



Gradual Discovery. In 1672 Sir Isaac Newton 

 made the simple but beautiful experiment (which 

 Kepler had also tried less effectively) of using a 

 prism to split up a ray of sunlight which entered 

 a darkened room through a round hole bored in the 

 shutter. He thereby produced a spectrum or image 

 of the differently coloured constituents of light, due, 

 as he showed, to the fact that these constituents (rays 

 of different wave-length, as we now say) have differ- 

 ent refrangibilities. This was the beginning of the 

 analysis of sunlight, which was destined to have such 

 a remarkable future. 



The historians tell us that a young Scotchman 

 Thomas Melvil (d. 1753) began the study of the 

 spectra of salts, and the spectroscope was certainly 

 a chemist's instrument before its astronomical value 

 was recognised. It may be recalled that several 

 elements caesium, rubidium, thallium, indium, gal- 

 lium, and scandium were discovered by means of 

 the spectroscope. In 1802, Wollaston replaced " the 

 round hole in the shutter " by a fine slit parallel 

 to the edge of the prisms, showed that there were 

 gaps in the solar spectrum, and made the further im- 

 portant step of contrasting the spectrum of sunlight 

 with that of a candle flame. 



Mechanical improvements were soon introduced 

 by Fraunhofer (1814) and Simms (1839). Fraun- 

 hofer, independently of Wollaston, also mapped out 

 a large number of the dark lines in the spectrum of 

 sunlight, and called particular attention to the fact 



