﻿398 Dr. Genevieve V. Morrow on Displacements in 



used, the strength being varied between 4 and 18 amperes 

 by means o£ resistances. The alternating current which was 

 used for the spark was furnished by a primary current of 

 25 amps, at 110 volts, which was transformed to a potential 

 of 10,000 volts. In parallel with the spark was a Franklin 

 condenser of 750 m. capacity. The distance between the 

 electrodes in both arc and spark was from 4 to 5 mm.; and 

 an enlarged image of the source of light was thrown on the 

 slit by means of a Schumann condenser, which consisted of 

 two crossed cylindrical quartz lenses — one being vertical and 

 the other horizontal. 



For obtaining the photographs Schleussner's orthochromatic 

 Viridin plates were used. They were 30 cm. long and about 

 4 cm. wide and were placed in a curved position in the 

 camera, so that they were on the circumference of a circle 

 of 2 '28 m. diameter. In this way the lines of the spectrum 

 along the whole length- of the plate were equally sharp. On 

 each plate either the arc or the spark spectrum of the sub- 

 stance examined was photographed, and immediately below 

 it and slightly overlapping it was taken the photograph 

 of the arc spectrum of iron to be used as a standard. A 

 vertical screen with a horizontal opening was placed before 

 the camera so that only part of the plate was exposed at first, 

 and it was then lowered the necessary distance before the 

 iron was photographed, precautions having been taken that 

 no possible displacement of the camera could occur between 

 the two exposures. There were always common lines in 

 both spectra due to some impurity, which could be compared 

 in order to see if any accidental displacement had occurred, 

 but in no case was such observed. The time of exposure for 

 the arc spectra varied between 2 and 5 seconds, and in the 

 spark spectra it reached a maximum of 5 minutes. The 

 plates were developed with hydroquinone. 



The wave-lengths of the standard iron lines were taken 

 from Rowland's tables, and were so chosen that each line of 

 the substance investigated could be referred to four w r ell 

 defined standard lines, that is to two on either side of it. 

 Each plate was measured ten times, — five times forwards 

 and five times backwards, and the mean of these measure- 

 ments taken to calculate the wave-length. Two values were 

 obtained for the wave-length of each line, one from each 

 pair of standards near it, and the mean of these two values 

 was taken as being correct. The difference between these 

 values was in most cases only a few thousandths of an Angstrom 

 unit, but in a few instances it reached 0*025 A.U., that is 



