Progress and Problems of Solar Physics. 133 



satisfactory explanation of the displacement of spectral 

 lines in the case of the prominences; but future re- 

 searches at a time of sunspot maximum will probably 

 be necessary to confirm the theory. 



In the year 1891 Professor Hale, of Chicago, was 

 first enabled to secure a photograph of a solar promi- 

 nence in full daylight by means of his spectroheliograph, 

 which consisted of a powerful spectroscope with two 

 automatically moving slits, one at the collimator and 

 the other at the eye-end of the view telescope. These 

 slits are so adjusted as to only admit the H or K line 

 light of calcium to the photographic plate while the 

 image of the sun is slowly drifting across the first slit 

 plate. By this means photographs of the prominences, 

 chromosphere and faculae were taken daily and are 

 available for study at leisure at any time. More re- 

 cently the great Rumford spectroheliograph attached to 

 the gigantic forty-inch telescope of the Yerkes observa- 

 tory in the hands of Professor Hale has yielded most 

 important results, and astronomers look forward to 

 future researches with this giant instrumental equip- 

 ment with great interest. 



The sun's chromosphere presents an interesting 

 bright line spectrum, consisting chiefly of hydrogen, 

 helium and calcium. Professor Young gives a list of 

 eleven lines which are always observable in the chromo- 

 sphere, three being due to helium, five to hydrogen, two 

 to calcium and the Corona line, and a list of thirty-four 

 others which are more or less conspicuous at certain 

 times, and under perfect conditions of seeing; these com- 

 prise in addition to the elements mentioned iron, 

 barium, sodium, titanium, chromium, magnesium, man- 

 ganese and strontium. During the eclipse visible in 

 India in January, 1898, the elements scandium and 

 gallium were added to the chromospheric list, and in 

 September, 1897, Professor Hale discerned carbon bands 

 at the edge of the sun, from which he inferred th,^ 

 existence of carbon vapor as a solar layer; this envelope 

 may be from 500 to 1,000 miles thick. Professor Young's 

 latest views on the reversing layer and chromosphere 

 are that they are "simply the uncondensed vapors and 

 gases which form the "atmosphere" in which the clouds 



