﻿certain Spectral Lines of Zinc and Titanium. 397 



oE the pitch were made in both directions along each centi- 

 metre o£ the screw, in order to avoid personal errors in the 

 readings. The values found in seven sets of measurements 

 varied between 0*4985 mm. and 0'4995 mm., so that 

 0*499 mm. was considered as being correct. 



For the present work it was necessary to know the value 

 of one turn of the screw in Angstrom units. To ascertain 

 this Professors Exner and Haschek kindly lent me a photo- 

 graphic plate of the spectrum of palladium, which had very 

 sharply defined lines. The iron spectrum was on this plate 

 as a standard, and the wave-lengths were taken from Row- 

 land's tables. The palladium lines as well as 32 standards 

 were measured, and the value of one turn of the screw in 

 Angstrom units was found from the standards. A curve was 

 drawn taking the mean wave-length of each pair of standards 

 as abscissa, and the corresponding factor for the screw as 

 ordinate. This gave 1*3868 as a mean value for the factor. 

 To ascertain how accurate the readings had been, the wave- 

 lengths of the measured palladium lines were calculated. 

 The differences between those found from this one measure- 

 ment and those usually accepted as being correct did not 

 exceed a few hundredths of an A.U. To test the accuracy 

 of the readings farther, the standards were measured in 

 both directions, and the mean error for each reading was 

 found to be 0*02038 A.U., which agrees very well with that 

 obtained above. From five measurements in each direction 

 for the same two lines, the mean error in one reading was 

 found to be 0*0225 A.U. 



From the above results it was evident that the micrometer 

 was sufficiently accurate to detect a displacement of 

 0*04 A. IT., which is the smallest recorded by Exner and 

 Haschek. 



The spectral apparatus consisted of a Rowland concave 

 grating whose radius of curvature was 4*56 m.; it had 

 20,000 lines to the inch and 70,000 over the whole divided 

 surface. The mounting of slit, grating, and camera was 

 that of the well known arrangement of Rowland. The 

 camera and grating are firmly bound together by a beam the 

 length of which is equal to the radius of curvature of the 

 grating, and they move on carriages along two rails which 

 are set at right angles to each other, and above the junction 

 of which the slit is situated. 



The electrodes were held in clips which were so arranged 

 that during the passage of the current they could be moved 

 sideways, or their distance apart could be altered. For the 

 production of the arc a direct current from 110 volt^ was 



