344 SMITHSONIAN misce;llane;ous collections vol. 52 



above the photosphere. Is the temperature within the spot low 

 enough to permit these substances to combine? 



For many years the spot spectrum had been known to contain a 

 number of bands and of faint Hues, but none of these had been 

 identified. Fortunately, the photographs obtained with the Snow 

 telescope show these bands far better than they can be seen visually, 

 and bring to light many new bands and thousands of faint lines of 

 unknown origin. Fig. i, Plate xxx, illustrates a comparison of one 

 of the red titanium oxide bands, made up of a great number of fine 

 lines terminating in three distinct heads, with the corresponding 

 region in a photograph of the spot spectrum. It will be seen that 

 practically all of the lines of the band photographed in the flame 

 of the electric arc are present in the spot. As many other titanium 

 bands have been found on the photographs, we now know not only 

 that many hundreds of the spot lines can be accounted for in this 

 way, but also that the hypothesis of reduced temperature is partially 

 confirmed. This identification of the titanium oxide flutings is due 

 to Adams. Soon after its publication, Fowler, of London, found 

 some of the bands in the green portion of our photographic map to 

 be due to magnesium hydride, another compound capable of with- 

 standing high temperatures. Still later, Olmsted discovered in our 

 Mount Wilson laboratory that certain spot bands in the red are due 

 to calcium hydride. He is continuing the search for other com- 

 pounds with improved apparatus in our new Pasadena laboratory. 

 The investigation may be an extensive one, because the spectra of 

 only a few of these compounds, which are formed at the high tem- 

 perature of the electric furnace, have hitherto been observed. Even 

 in these cases no large scale photographs, or sufficiently accurate 

 measurements of the lines, have been published. 



The presence of compounds in spots appears favorable to the 

 hypothesis of reduced temperature, though it does not settle the 

 question beyond doubt. It next became interesting to inquire 

 whether analogous conditions could be found among the stars. As 

 already remarked, the stars are so distant that their images in the 

 most powerful telescopes are mere needle points, so that objects like 

 Sun-spots, if they exist on the stars, cannot be observed. Accord- 

 ing to current ideas of stellar evolution, the stars pass through a 

 long process of development, during which their temperature, per- 

 haps comparatively low in the embryonic stage represented by the 

 condensing nebula?, reaches a maximum in the white stars, and then 

 declines during the period of old age exemplified in the red stars. 

 If, then, a Sun-spot is a mass of solar vapors reduced somewhat in 



