116 THE PATH OF SCIENCE 



with the positions of the stars and with the movement of the 

 planets in tlie solar system. Astronomers spent the greater 

 part of their time in the computation of positions and in the 

 verification of their results. The greatest triumph of that 

 period was the calculation of the existence and orbit of a 

 planet beyond Uranus, a result obtained from slight devia- 

 tions between the observed position of Uranus and that which 

 was calculated from the influence of the other planets, and 

 the verification of this discovery by the observation of 

 Neptune when the telescope was directed to the calculated 

 position. 



On the nature of the stars and the constitution of the 

 stellar universe, there was much speculation, but few facts 

 seemed to be obtainable. As an example of a thing that 

 must forever remain unknown, August Comte quoted the 

 chemical composition of the heavenly bodies. All this ^\as 

 changed by the application of the spectroscope to astronomy. 

 Von Fraunhofer had observed that in the spectrum of the 

 sun there were black lines, and Robert Bunsen and Gustav 

 Kirchhoff showed that these corresponded in position to the 

 bright lines in the emission spectra of some of the elements. 

 One of these was so unmistakable that its identification was. 

 certain— the double line in the yellow, to which was assigned 

 the letter D by von Fraunhofer, corresponding exactly to 

 the double emission line of sodium in the yellow. Jules, 

 Janssen and Norman Lockyer, pioneers in astronomical spec- 

 troscopy, observed in the spectrum of the chromosphere a 

 bright yellow line slightly on the gi^een side of the D line,, 

 which they ascribed to an unknown element; and Lockyer,. 

 greatly daring, named this element from the sun, helium.. 

 In 1896 AVilliam Ramsay, who had identified argon in the 

 earth's atmosphere,* w^as looking for argon in the gas oc- 

 cluded in certain minerals when the spectroscope showed hiixt 

 that the long-sought-for helium had been found.. 



* Chapter VI, p. 134. 



