130 PROGRESS IN ASTRONOMY. 



explained by Stokes and others, as before stated. The lines in the 

 solar spectrum were mapped with great diligence b}^ Kirchhoff in 1861 

 and 1862, and later by Angstrom and Thalen, and this was done side 

 by side with chemical work in the laboratory. The chemistiy of the 

 sun was thus to a great extent revealed. It was no longer a habitable 

 globe, but one with its visible boundary at a fierce heat, surrounded 

 by an atmosphere of metallic vapors, chief among them iron, also in a 

 state of incandescence. To these metallic vapors Angstrom added 

 hydrogen shortly afterwards. 



Here, then, was established a firm link between the heavens and the 

 earth; the first step to the problem of the chemistry of space had been 

 taken. 



It was only natural that as advances were made the instrumental 

 equipment should keep pace with them. Spectroscopes were built on 

 a larger scale; more prisms, which meant greater disperson, were 

 employed to render the measurements of the lines in spectra more 

 accurate. The growth of our knowledge especially necessitated the 

 making of maps of the lines in the solar spectrum, and in the spectra 

 of the chemifcal elements which had been compared with it on a 

 natural scale. This was done by Angstrom, who utilized for this 

 purpose the diffraction grating invented by Fraunhofer, and defined 

 the position of all lines in spectra by their ''wavelengths,'" in ten- 

 millionths of a millimeter or "tenth-meters." 



In 1862 Rutherfurd extended Fraunhofer's work on the stars by a 

 first attempt at classification. Two years later Huggins and Miller 

 produced maps of the spectra of some stars. Donati demonstrated 

 that comets gave radiation spectra and Huggins did the same for 

 nebuke. 



By these observations comets and nebulas were shown to be spectro- 

 scopicall}" different from stars, which at that time were studied by their 

 dark lines onlv- 



Chiefly by the labors of Pickering, the energetic head of the Harvard 

 Observatory, science has been enriched during the later years by 

 observations of thousands of stellar spectra, the study of which has 

 brought about the most marvelous advance in our knowledge. 



These priceless data have enabled us now to classify the stars not 

 only b}" their brightness, or their color, but by their chemistry. 



Next to be chronicled is the application of the so-called Doppler- 

 Fizeau principle, which teaches us that when a light source is approach- 

 ing or receding from us the light waves are crushed together or drawn 

 out, so that the wave length is changed. The amount of change 

 gives us the velocity of approach or recess, so that the rate of move- 

 ment of stars toward or from the earth, or the upriLsh or downrush of 

 the solar vapors on the sun's disk, can be accurately determined, A 

 further utilization of this principle is found when the stars are so close 



