OF ARTS AND SCIENCES. 3 



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 crit- 

 ically 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 2,000 revolutions per min- 

 ute is connected to a commutator of four segments upon a fixed spindle, 

 around which revolve two pairs of brushes. The result of this com- 

 bination is that the current is very frequently 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 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 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. 



The points of aluminium being permanently fixed in front of the 

 slit, sunlight is introduced, the camera brought to focus once for all, 



