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 ISTewton 

 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 — ca?sium, 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 



