﻿certain Spectral Lines of Zinc and Titanium. 395 



some of the lines by means of a micrometer. The present 

 work has been undertaken to ascertain if under these con- 

 ditions there is any displacement in the lines. 



Rowland observed that the position of the iron lines in 

 the sun's spectrum was not always constant, and the subject 

 was farther studied by Jewell *, who found marked dif- 

 ferences between the positions of the metallic lines in the 

 arc and the sun. Humphreys and Mohler f noticed that 

 increase of pressure in the arc caused metallic lines to be 

 displaced towards the red, but that variation in the strength 

 of the current did not affect the position of the reversals of 

 lines. Mohler J found corresponding results on lowering the 

 pressure, but Duffield§ noticed that reversed as well as bright 

 lines were displaced towards the red, under increase of pres- 

 sure in the arc. Jewell || found that the stronger reversed 

 lines were those whose displacement was greatest, and also 

 that an increase in the arc in the quantity of material pro- 

 ducing the line always displaced it towards the red, but an 

 increase in the quantity of other material did not change the 

 position of the line to the same extent, if at all. KentlJ com- 

 pared the positions of the arc and spark lines with each other, 

 and found that the part of the spark near the terminals gave 

 lines the wave-lengths of which were greater than those of 

 the arc, whilst those produced at the centre, where there is 

 very little pressure, were not displaced, or only to a very 

 slight extent. He suggests this as a reason for the fact that 

 Eder and Valenta have observed no real displacements when 

 comparing arc and spark spectra. 



The question of the displacement of spectral lines is dis- 

 cussed by Exner and Haschek in their book ' Die Spektren 

 der Elemente bei normalen Druck/ and their results show 

 that the more intense a line is, the more strongly is it 

 displaced towards the red. They found displacements of 

 considerable dimensions in both arc and spark spectra, and 

 in the bright as well as in the reversed lines. In another 

 publication** they examined the spectra of the elements 

 potassium, tin, and zinc under various conditions, and obtained 

 displacements in the lines which in some cases were more 

 than 0*1 Angstrom unit, far beyond the region of error in 



* Jewell, Astropliys. Journ. iii. (189G). 

 t Humphreys and'Moliler, Astropliys. Journ. iii. (1S9G). 

 I Mohler, Astropliys. Journ. iv. (1S96). 

 § Duffield, Astropliys. Journ. xxvi. (1907). 

 || Jewell, Astropliys". Journ. iii. (1896). 

 «ft Kent, Astropliys. Journ. xvii. (1903) ; xxii. (1905). 

 ** Exner and Haschek, Sitmngsber. der Wien. Akad. cxv. II. A. (1906). 



