EFFECT OF PRESSURE UPON ARC SPECTRA. NICKEL. 211 



MITCHELL* attributes the occurrence of enhanced lines in the chromospheric spectrum 

 partly to the reduction of pressure consequent upon the greater altitude and partly 

 to the presence of hydrogen there. This will be further discussed in a subsequent 

 paper. 



7. The Displacement of the Lines: (l) The Measurement of the Plates. The bulk 

 of the measurements were made by Mr. F. E. PEARSE, for whose assistance I am 

 indebted to the Government Grant Committee. The photographs were placed in the 

 fixed carrier of a modified Hilger photo-measuring machine in which the movable 

 part was the microscope which was controlled by a screw whose drumhead reading 

 could be estimated to the thousandth part of a millimetre. In order that lines of 

 various breadths could be measured the microscope could be converted into a telescope 

 and a considerable range of magnification achieved. There were two pairs of parallel 

 wires of different intervals in the eye-piece, either of which could be set perpendicular 

 to the direction of travel of the slide and parallel to the spectrum lines. This latter 

 operation had been found difficult in previous Avork, so the later photographs had been 

 taken with a shutter, in which short slits had been cut to allow the top and bottom 

 of the comparison lines to affect the photographic plate above and below the central 

 strip. In each 50 cm. there were three such extra pairs of slits, each about 2 cm. long, 

 e.g., lines 15, 16, 17, 18, Plate 1. It was always possible to find one at least of these 

 in the range of spectrum upon the measuring machine, and the parallelism of the 

 cross wires was consequently attained with ease and considerable accuracy. Two 

 readings were taken with the plate placed with the red on the right-hand side, the 

 setting being first on the upper and then on the lower half of the line under pressure ; 

 the plate was then reversed and the readings repeated. Four readings were thus 

 invariably taken. In many instances others were made. The readings were checked 

 by the writer, who made a point of measuring eacli line at some one pressure. As a rule 

 these readings were not included in the mean results, because there was some personal 

 equation in the measurements, and it seemed best to have a homogeneous set of 

 readings made by one individual because these are then more strictly comparable 

 with one another. It is interesting to note that even though different observers may 

 obtain different absolute values for the displacements upon a single photographic plate, 

 there is usually agreement between the relative values of their measurements of the 

 displacements of different lines. For instance, the groupings are usually the same and 

 also the ratios of the mean displacements of the groups. 



The order of accuracy obtained is shown by the following, Table II., in which a 

 few readings taken at random are reproduced. They illustrate the agreement of 

 Mr. PEARSE'S readings amongst themselves and with those of the writer. 



* MITCHELL, ' Astrophysical Journal, vol. XXXVIIL, p. 407, 1913. 



