﻿396 Dr. Genevieve V. Morrow on Displacements in 



the measurements. The displacements increased with the 

 increase of the density of the glowing vapour. By a com- 

 parison of the arc and spark spectrum of the same element, 

 they found that the wave-lengths in the spark were often 

 less than those in the arc, but on the other hand the maximum 

 displacement in the spark was the larger, which the authors 

 attributed to the greater difference of density between the 

 central and outer layers of the spark. 



Amongst those who have found no displacement in spectral 

 lines are Eder and Valenta *. They compared directly on 

 the photographic plate the eight lines of zinc, in which Exner 

 and Haschek had found displacements between arc and sparky 

 but they could not observe any alteration in the position of 

 the lines with respect to each other. They are of the opinion 

 that an apparent displacement may be caused in a line by 

 unsym metrical broadening, but that the position of maximum 

 intensity remains unaltered, and that the so-called displace- 

 ments obtained by observers are caused by the measurements 

 not being made through the most intense part of the line. 

 Kayser t also has found no displacements in spectral lines,. 

 and suggests that the results of Exner and Haschek and 

 those of Kent have been caused either by errors in measure- 

 ment or by bad adjustment of the apparatus. 



The spectral apparatus which was employed in the present 

 investigation Avas the same as that used by Exner and Haschek 

 in their experiments. In order to avoid the possibility of 

 errors which might be caused by the projection method of 

 measurement used by them, and of which Kayser did not 

 approve, the plates were measured by means of a micro- 

 meter. This micrometer was constructed by Perreaux and 

 permitted of measurements extending 30 cm. horizontally. 

 It was of great importance to know if the thread of the 

 screw was sufficiently fine and constant to permit of its 

 being used in the present investigation, so the pitch wa* at 

 first ascertained by the usual method. The distance travelled 

 by the microscope along the screw was measured by focussing 

 the cross-hairs on a divided millimetre scale of plated brass 

 supplied by the Societe Genevoise. This scale had been 

 tested in Paris and found correct. Several sets of measure- 

 ments were made and curves drawn to ascertain if the screw 

 possessed periodic errors. The latter were not found, and 

 the mean difference between the measurements of the pitch 

 was *0005 mm. Further measurements for finding the value 



* Eder and Valenta, Sitzungsber. dcr Wien. Akad. cxii. II. A. (1903). 

 f Kayser, Zeitschr.fiir wiss. Photographic, iii. (1905). 



