EFFECT OF PRESSURE UPON ARC SPECTRA. 209 



under pressure (j, p, Plate 11). No relation has l)een found between the original 

 intensity of a line and its width under pressure. 



0. The magnitude of the broadening even for the well-defined lines may be as great 

 as 8 A.U. under 100 atmospheres. 



7. The types of broadening of the nebulous and sharp non-series lines are very 

 similar, but the latter are more sharply defined on their violet edges. They retain 

 their characteristic " soft" and " hard" appearances throughout. 



8. In the neighbourhood of the 1st sub-series lines there is generally a cloudy 

 appearance under pressure, as though there is some tendency of the vibrating system 

 producing these lines to form a banded spectrum. This resembles in a modified 

 degree the spectrum of the silver arc, most of whose lines vanish under pressure, 

 giving place at low pressures to a banded or nebulous fluted spectrum, and at the 

 highest pressure reached by the writer (200 atmospheres*) to a practically continuous 

 spectrum. 



6. THE DISPLACEMENT OF THE LINES. 

 (1) Method of Measurement of the Photographs, 



The Kayser Measuring Machine was used, the setting being always made between 

 parallel threads as accurately as possible upon the most intense portions of the lines 

 under pressure, and advantage was taken of the astigmatic property of the grating of 

 narrowing a line at its extremities. Twelve settings were made upon each line on 

 each plate, six with the plate in one position and six with it in the reversed position. 

 When there was not good agreement between the readings this number was exceeded. 

 The fuzziness and great breadth of the lines at 100 atmospheres made the setting ot 

 the wires a matter of difficulty it was found simplest to prick the most intense part 

 of each line upon the film before placing the photograph in the machine. 



(2) Description of Table I. 



Table I. shows the nature of the agreement obtained from the measurements of the 

 same line on different photographs taken at the same pressure. 



The first column indicates the line, and the subsequent columns its displacement 

 upon the different plates named at the head of the column, the mean values for each 

 pressure being also given. The readings are in thousandths of a turn of the screw 

 of the measuring machine, whose pitch is ^ mm. The drum-head is divided into 

 100 divisions. 



The agreement is remarkably good at high pressures, but at low pressures the 

 width of the line has increased in a greater proportion than the displacement, and the 

 concordance is not so great. At 5 and 10 atmospheres there is little agreement 



between the plates. 



* Added October 23, 1908. G. D. 



VOL. CCTX. A. 2 E 



