THE GROWTH OF OUR KNOWLEDGE OF HELIUM. 269 



telescope was fitted with a high power micrometer eye- 

 piece ; the dispersion was sufficient to easily show the 

 difference of position of the D 3 line on the east and west 

 limbs, due to the sun's rotation. Observations of the 

 chromosphere were therefore confined to the poles. 



During the short time that the tube retained its great 

 brilliancy, a faint line, a little less refrangible than the 

 bright orange one, and making a close double with it, was 

 readily seen ; but afterwards a sudden change took place, 

 and the lines almost faded away. While the gas line was 

 brilliant, it was found to be " the least trace more refrangible 

 than D 3 , about the thickness of the line itself, which was 

 but narrow" ("Observatory Note Book"). The sudden 

 diminution in the brightness of the lines made subsequent 

 observations less certain, but the instrumental conditions 

 being slightly varied, it was thought that the gas line was 

 probably less refrangible than the D 3 line by about the 

 same amount that the first observation showed it to be 

 more refrangible. Giving the observations equal weight, 

 the gas line would thus appear to be probably coincident 

 with the middle of the chromospheric line, but if extra 

 weight be given to the first observation, made under much 

 more favourable conditions, the gas line would be slightly 

 more refrangible than the middle of the chromosphere line. 



Pressure of other work did not permit the continuation 

 of the comparisons. In the meantime, Runge and Paschen 

 announced (Nature, vol. Hi., p. 128) that they also had seen 

 the orange line of the cleveite gas to be a close double, 

 neither component having exactly the same wave-length as 

 D 3 , according to Rowland. 



They give the wave-length of the brightest component as 

 5878*883, and the distance apart of the lines as 0*323. 



This independent confirmation of the duplicity of the 

 gas line led me to carefully re-observe the D 3 line in the 

 chromosphere for evidences of doubling. On 14th June 

 observations were made by Mr. Shackleton and myself of 

 the D 3 line in the third and fourth order spectra under 

 favourable conditions ; " the line was seen best in the fourth 

 order, on an extension of the chromosphere or prominence 



