DIFFERENCE IN WAVE-LENGTHS OF TITANIUM 

 AA 3900 AND 3913 IN ARC AND SPARK. 



By Nobton a. Kent and Alfred H. Avery. 



Presented by J. Trowbridge October 9, 1907. Received October 9, 1907. 



In June, 1905, one of the writers of the present paper published the 

 results of a careful series of experiments dealing with the variation 

 in the wave-length of certain lines of the spark spectra of titanium, 

 iron, and zinc with the electrical conditions of the discharge. ■•■ Sub- 

 sequently Keller, working under Kayser, published a paper ^ in which 

 the suggestion was made that the apparent non-coincidences of the 

 spark and the comparison arc lines were due to the fact that the slit 

 was not accurately adjusted to parallelism with the grating ruling; 

 and the statement was made that the plumb-line method of adjustment 

 employed by the writer was of less delicacy than the spectroscopic. 



The substance of Keller's explanation of the manner in which shifts 

 could be introduced by orientation of the spectrometer slit is as 

 follows: Given a perpendicular grating ruling, an astigmatic instru- 

 ment such as the concave grating will give a perpendicular line image 

 for every point of the line source as object. If, then, the line source 

 or slit be at an angle (say clockwise as one faces it) with the grating 

 ruling, each spectral line will be a composite of lines arranged as in 

 Figure 1. 



The result will be an image which is apparently rotated in the direc- 

 tion of the slit. If, then, on one photographic plate two exposures be 

 made, one each of arc and spark, and the position of the adjacent tips 

 of the images of any spectral line be measured by a comparator, any 

 displacement desired may be introduced by a rotation of the slit. 



But Keller's explanation does not apply to the method of exposure 

 employed by the writer of the former paper — a method of triple 

 exposure, two of the arc (the first and the third) superimposed hori- 

 zontally but not wholly vertically and spanned by the spark exposure, 

 as in Figure 2. 



* These Proceedings, 41, No. 10, July, 1905. 



' Ueber die angebliche Verschiebung der Funkenlinien. Inaugural-Disser- 

 tation Christian Keller. 1906. 



VOL. XLIII. — 23 



