314 the president's address. 



observed to cross the spectrum of solar and stellar light ; and, 

 in like manner, across the spectra of the heated vapours 

 of terrestrial metals and gases, the spectrum of each star, 

 metal, or gas, having its own peculiar system of lines, the 

 relative positions of which in each spectrum are invari- 

 able. By examining the corresponding position of these dark 

 or bright lines in different spectra, it is possible to name 

 many of the substances existing as gas in the sun and stars. 

 You are doubtless all aware that when a beam, or point, of solar 

 light is made to pass through an ordinary prism, it is resolved, 

 or decomposed, into a number of divergent rays of different 

 colours and refrangibilities, which if projected on a screen, 

 forms not a beam of white light as before entering the prism, 

 but a luminous band exhibiting the seven well-known prismatic 

 colours, passing from red, or the ray least refracted or bent, 

 through yellow, green, blue, violet, indigo, and lavender. The 

 red end of the spectrum contains the heat rays, and the lavender 

 end the actinic and chemical rays, thus accounting for the three- 

 fold action of the sunbeam, its heating, lighting, and chemical 

 powers. 



Dr. Wollaston, in 1802, was the first observer of some of 

 the principal dark lines in the solar spectrum. He did not, 

 however, consider that his discovery was of much scientific im- 

 portance, except that he formed an opinion that they were 

 caused by actual and generic lines of separation between the 

 distinctive colours of the spectrum. Thirteen years afterwards, 

 Fraunhofer, a distinguished German physicist, while viewing a 

 distinct and narrow line of sunlight, by placing a prism of great 

 purity before the object-glass of a small telescope, observed and 

 accurately measured the relative positions of a large number of 

 dark lines which had escaped Dr. "Wollaston. More recent 

 physical astronomers have mapped out the lines in the solar 

 spectrum with still greater completeness, and have micro- 

 metrically measured, with extreme accuracy, the relative positions 

 of thousands of lines. The position of each line in the spectrum 

 is usually expressed by a number measured on a scale of wave- 

 lengths, affording an easy method for referring the dark or 

 bright lines in the spectra of the stars and metals, to the corres- 

 ponding lines in the solar spectrum. 



