i6 



thence to a lens of 18 feet focal length, which 

 rendered the rays parallel; they then met a grating 

 of polished metal, ruled with some 15,000 lines to 

 the inch; this analyzed the composite light into 

 its constituent parts and returned the rays through 

 the lens, which formed an image of the long spec- 

 trum band on a photographic plate below the slit. 

 With this spectrograph, constructed in our shop 

 in Pasadena and mounted for use with the Snow 



FIG. g. Direct photograph of Sun-spot. 



telescope, it soon became an easy matter to photo- 

 graph sun-spot spectra. The curious widened 

 lines, the much debated bands, and the strength- 

 ened and weakened lines were thus accurately 

 recorded for study. What is the cause of these 

 peculiarities? The first step was to test by labor- 

 atory experiments the hypothesis that some of 

 them are due to reduced temperature of the spot 

 vapors. In studying such phenomena the spec- 

 troscopist is in a position much like that of an 



