KENT AND AVERY. — WAVE-LENGTHS OF TITANIUM. 355 



the above mentioned criticism could apply, it seemed advisable to 

 test the matter, and the following experiments were undertaken to 

 decide the two following questions: 



(1) Is the plumb-line method of adjustment of slit and grating rul- 

 ing to parallelism more or less accurate than the spectroscopic 1 



(2) Will an orientation of the slit introduce a shift if the triple 

 method of exposure be used 1 



Conditions of Experiment. 



The conditions under which the present work was carried on were, 

 as far as possible, those of the previous series of experiments. By the 

 courtesy of Professor Trowbridge and Professor Sabine every facility 

 of the Jefferson Physical Laboratory was placed at our disposal. The 

 grating — a 6" Rowland concave, of 20,000 lines to the inch and 21 -foot 

 radius of curvature, an excellent instrument — was kindly loaned by 

 Professor Trowbridge, and the mount was that belonging to the labora- 

 tory and located on the third story of the building. The beams were 

 heavy timbers supported wholly from the walls of the building. The 

 slit, grating holder, camera-box, rheostat, transformer, and condenser 

 were those used in the former work. The usual precautions relative 

 to temperature changes were taken, the whole "mount being wrapped 

 in several layers of newspaper. The vibrations of the building due to 

 wind and heavy machinery necessitated working at times when these 

 disturbing influences were absent. All plates not showing horizontal 

 coincidence of the arc exposures were rejected. The current used for 

 both arc and spark was the 110 volt, 66 cycle alternating current of 

 the Cambridge Electric Light Company. The frequency of the current 

 used in the previous work was 133, but as the transformer was built 

 for 66 cycles no difficulty was experienced in this regard. The volt- 

 meter, ammeter, and wattmeter were of Thompson form, and of ranges 

 — 65 volts ; — 60 amperes ; and — 45 hecto- watts, respectively. 

 Thus the conditions were the same as those formerly employed in all 

 respects but location, frequency of current, and grating. 



Results obtained. 



(1) Relative merits of plumb-line and spectroscopic methods of adjust- 

 ment. The grating holder was fitted with two opposing screws moving 

 in a horizontal direction and controlling the orientation of the grating. 

 It was found by trial that by the unaided eye the parallelism of either 

 end of the ruled space of the grating with the silk thread of a plumb- 

 line suspended from the grating holder could be adjusted so that the 



