Trowbridge and Hutohins — Oxygen in the Sun. 265 



I express here ray admiration for the discovery of Mr. Draper, 

 and I hope that his results, so well confirmed by the photo- 

 graphic proofs that our learned member, M. Cornu, has shown 

 the Academy, will not delay in being universally accepted, by 

 competent judges." 



The opinion thus expressed by so eminent an authority as M. 

 Faye testifies to the strength of the evidence brought forward 

 by Dr. Draper. With the exception of Prof. John C. Draper, 

 physicists, in so far as they have expressed their views, have 

 generally accepted the hypothesis of Dr. Draper. No one, to 

 our knowledge, has critically examined the hypothesis of 

 bright lines in the solar spectrum. 



The reader of Dr. H. Draper's account of his experiments 

 will remember the difficulties he encountered in obtaining an air 

 spectrum, of sufficient brightness to record itself upon the pho- 

 tographic plate. The time that has elapsed since his work does 

 not seem to have made those difficulties less, and, in spite of all 

 that our ingenuity has been able to devise, we have been practi- 

 cally confined to taking the spark in free air or oxygen at 

 atmospheric pressure, notwithstanding the broad and hazy 

 character of the lines under these conditions. 



Not to record a long list of failures extending over several 

 months, we will briefly describe the arrangements in their final 

 form. 



An alternating current dynamo driven at 2,000 revolutions 

 per minute is connected to a commutator of four segments 

 upon a fixed spindle, around which revolve two pairs of brushes. 

 The result of this combination is that the current is very fre- 

 quently and sharply interrupted. This interrupted current is 

 used to excite three large quantity coils connected in series. 

 From two to twelve jars were employed as a condenser to the 

 secondary current. The spark was taken between two stout 

 rods of aluminium placed immediately in front of the slit, and 

 the spark passed, between them with a deafening rattle, and 

 gave about the light of two candles. We tried Dr. Draper's 

 device of a soapstone compressor for the spark, but in our 

 hands the walls of the soapstone near the spark melted down, 

 and formed a conducting surface over which the current passed. 



The photographic apparatus is the large instrument of Pro- 

 fessor Eowland, — a concave grating with ruled surface 6x2 

 inches, mounted upon an iron girder 23 feet long, moving upon 

 two tracks at right angles, as has been previously described by 

 him and others. Sunlight is introduced by a heliostat with 

 mirror silvered on first surface, and an image of the sun 

 formed on the slit by means of a quartz lens of five feet focus. 

 The method of working with the apparatus so arranged has 

 been as follows. 



