﻿382 



Prof. H. Crew and Mr. R. Tatnall on a 



a hundred-volt circuit and an alternating current of from two 

 to ten amperes. Higher voltages sustain a longer arc, and 

 thus protect the metal from mechanical wear. 



For the purpose of a comparison spectrum is used a second 

 counter-shaft placed parallel to, and in the same horizontal 

 plane with, the first. This shaft carries an iron disk, about 

 an inch in diameter, against which is fed a piece of iron 

 tubing. The spectrum of any one metal having been photo- 

 graphed, the whole instrument is translated laterally and the 

 current switched on to the iron disk. While not so con- 

 venient as the sun in many ways, the iron spectrum has an 

 abundance of sharp lines evenly distributed : it permits one 

 to work in all kinds of weather and at night. 



The plates whose measures follow will illustrate the method. 

 They were taken with a Rowland concave grating of ten feet 

 radius and ruled with fifty thousand lines. The portion of 

 the plate measured, in each case, covers a part of the spec- 

 tram where the carbon bands are strong. Knowing of no 

 adequate method of reproduction, except silver printing, 

 which is too expensive, we have selected three typical plates 

 and simply measured on a dividing-engine all the lines visible, 

 including " ghosts " and recognized impurities. The tables 

 explain themselves. They include all the lines certainly 

 visible through the reading-microscope of the dividing-engine ; 

 but a still lower-power microscope shows a number of weaker 

 lines between those measured. The wave-lengths were de- 

 termined not with the highest accuracy possible, but well 

 within a tenth of an Angstrom unit, which is usually ample 

 for purposes of identification. The method was interpolated 

 between two of Rowland's standard iron lines, except in the 

 case of copper, where, for convenience, the interpolation is 

 between two of Kayser and Runge's copper lines. 



Plate No. 178. Tin. 







Kayser 





Element. 



Plate 178. 



and 



Remarks. 







Runge. 





Tin 



(4893-66) 



3262-44 



f Third order line : not completely ab- 

 \ sorbed by glass. 



Tin 



(4762-72) 



317512 





(Ghost)... 



4531-20 





Second order ghost of Sn 4524*92. 



(Ghost) ... 



4528-04 





First order ghost of Sn 4524*92. 



Tin 



4524-91 



4524-92 



Intensity 2. 



(Ghost) ... 



4521-77 





First order ghost of Sn 4524-92. 



(Ghost) ... 



4518-63 





Second order ghost of Sa 4524*92. 





4511-43 





Intensity 6 ; sharp. 



