40 



DR. W. GEOFFREY DUFFIELD ON THE 



structure is not apparent, either because it is not invariably present in the arc or 

 because the slit was too wide. Upon this photograph the reversals were rather better 

 defined than upon the reproductions. 



7. The DiitjiliKviiH'iit of the Lines: (l) Method of Measurement of the Photo- 

 ffraphs.llie 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. Thirty-six settings were made upon 

 each line on each plate, 18 with the plate in one position and 18 with it in the 

 reversed position. When there was not good agreement between the readings this 

 number was exceeded. 



(2) Table of the Displacements. The first column contains the arbitrary letters 

 assigned to the lines to facilitate reference to them. The second column gives the 

 wave-lengths according to KAYSER and RUNGE'S tables. The subsequent columns give 

 the displacements at the corresponding pressures, the use of italic figures, e.g., 0'220, 

 for displacements, indicates that the line is reversed. An asterisk (*) denotes that 

 the line is reversed but that measurements could not be satisfactorily made at that 

 pressure. In all cases the displacements are towards the side of greater wave-length. 

 The pressures are the excess above 1 atmosphere. Figures in brackets indicate that 

 the measurements were made with difficulty. 



TABLE I. 



[This table has been revised. December, 1910.} 



(3) The Relation between the Pressure and the Displacement. From the infor- 



s supplied by these measurements it is gathered that an approximately 



.ear relat.onsh.p holds between the pressure and the displacement, as it does in the 



and copper but the rates of displacement with the pressure are not 



.tmgu.shab , The higher rate belongs to the series lines a, c, and e, which 



apkced more than the non-series line d. 



'I.;,,,; , /Ma(,W Intemit y under Pre>mre.~lt has already been pointed 

 for hrc^dened .,, the - intensity is an indefinite term. If the intensity 



