158 HISTORY OF GEOLOGY AND PALEONTOLOGY. 



which a celestial body passes before and after its consolida- 

 tion. 



The differences in the colour and brightness of the fixed 

 stars suggested to the early astrologists that the stars differed 

 in their individual constitution. The catalogue of the 

 Ptolemaic Stellar Chart classifies the stars in six groups 

 according to their brilliancy. The attempt was frequently 

 made by Sir William Herschel among others to erect a more 

 precise system upon the basis of the intensity of the light 

 radiated from the different stars, but no satisfactory result was 

 obtained. The grouping of stars according to their colour 

 met with more success. The early astrologists distinguished 

 white, yellow, and red stars; in 1686 Mariotte observed blue 

 stars for the first time; and later, in 1782, Herschel observed 

 double stars displaying different colours. By means of the 

 spectroscope recent researches have arrived at an explanation 

 of the different brilliancy and colour of the fixed stars. 



The sun and all fixed stars have a continuous spectrum that 

 is interrupted by the dark lines of the vaporous substances in 

 the photosphere; the Fraunhofer lines are absent in the spectra 

 of planets, or bodies which have only reflected light Angelo 

 Secchi in his work on "the sun," in 1872, distinguished four 

 groups according to the spectroscopical character of the stars : 

 i, white and blue; 2, yellow; 3, orange-coloured and red; 

 4, blood-red. 



Secchi, Vogel, and Scheiner (1890) regard the differently 

 coloured stars as bodies representing different phases in the 

 cooling of nebulous masses. According to their investigations, 

 the white and blue stars are the brightest and hottest ; their 

 temperature is so high that the gases and metallic vapours in 

 their photosphere only exert a very slight absorptive power, 

 and the spectra are consequently either quite simple or show 

 extreme faint lines. The vast concourse of yellow stars are in 

 the farthest phase of condensation, which is represented by 

 the sun or central star of our system ; their spectra exhibit 

 numerous and powerful dark lines, indicating the presence of 

 several of the metals in addition to gases and metallic vapours. 

 The spectra of the red stars display broad dark streaks indi- 

 cative of metallic compounds, and it is inferred that the 

 temperature in those stars must be sufficiently reduced to 

 allow the metallic vapours in the atmosphere to enter into 

 various chemical combinations. The spectra of some of the 



