PHYSICAL SCIENCE IN THE NINETEENTH CENTURY 353 



Newton himself had pronounced the pure emission theory to be 

 insufficient and only a preliminary formulation. 



Young boldly generalized the undulatory theory by maintaining that 

 " a luminiferous ether pervades the universe, rare and elastic in a high 

 degree," that the sensation of different colors depends on the different 

 frequency of vibration excited by light in the retina. . . . 



In January, 1817, long before Fresnel had made up his mind to adopt 

 a similar conclusion . . . Young announced in a letter . . . that in 

 the assumption of transverse vibrations, after the manner of the vibra- 

 tions of a stretched string, lay the possibility of explaining polariza- 

 tion. ... Merz. 



THE SPECTROSCOPE AND SPECTRUM ANALYSIS. For closer 

 study of the spectrum of Newton and the " dark lines " observed by 

 Fraunhofer in 1815 (and in 1802 by Wollaston) in the spectrum, 

 Kirchhoff and Bunsen in 1859-60 perfected the "spectroscope." 

 This is essentially no more than a telescope so attached to the 

 prism producing the spectrum from a slit as to facilitate minute 

 scrutiny of the latter. It was by these workers and at this time 

 that spectrum analysis became firmly established as a means of 

 detecting the chemical constituents of celestial bodies. Kirch- 

 hoff wrote in 1859 : 



I conclude that colored flames in the spectra of which bright lines 

 present themselves, so weaken the rays of the color of these lines, 

 when such rays pour through them, that in place of the bright lines, 

 dark ones appear as soon as there is brought behind the flame a source 

 of light of sufficient intensity in which these lines are otherwise want- 

 ing, thus originating two great applications of his principle the 

 search, through the study of the spectra of distant stellar sources of 

 light, after the ingredients which are present in those distant lumi- 

 naries, and the search, through the study of the flames of terrestrial 

 substances, for new spectral lines announcing yet undiscovered ele- 

 ments. 



In 1862, only three years after Kirchhoff and Bunsen's application of 

 th spectroscope to the study of the sun, Huggins measured the posi- 

 tion of the lines in the spectra of about forty stars, with a small slit 

 spectroscope attached to an 8-inch telescope. In 1876 he successfully 

 applied photography to a study of the ultra-violet region of stellar 



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