304 Prof. Trowbridge and Mr. Hutchins on 



o 



ever may be the fate that the progress of spectrum analysis 

 reserves to them, I express here my admiration for the 

 discovery of Mr. Draper, and I hope that his results, so well 

 confirmed by the photographic proofs that our learned mem- 

 ber, M. Cornu, has shown the Academy, will meet with no 

 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 

 photographic plate. The time that has elapsed since his 

 work does not seem to have made those difficulties less, and, 

 in spite of all our ingenuity has been able to devise, we have 

 been practically 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 2000 revolutions 

 per minute was connected to a commutator of four segments 

 upon a fixed spindle, around which revolved two pairs of 

 brushes. The result of this combination was that the current 

 was very frequently and sharply interrupted. This interrupted 

 current was used to excite three large quantity- coils connected 

 in series. From two to twelve jars were employed as a con- 

 denser 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 

 Professor Rowland, — 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 previ- 

 ously 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 



