L.Bell — Absolute Wave-length of Light. 273 



then, if i is as before, the angle of incidence, 



A = s sin cp cos (i + 6) 



n 



sin i = sin(« + d) cos cp 



cos cp rv 



tan i — — — - o 



1 — cos cp 



In the fourth method also, i is retained, but given a definite 

 value. Putting the general formula in the form, 



A = 2 s sin -£. cos li — J- 



n 2 V 2 



the deviation represented by the angular term will evidently 

 be a minimum when i = — . If then one observes in the posi- 

 tion of minimum deviation, 



A = — 2 s sin -L- 



72 2 



In the fifth method collimator and. observing telescope are kept 

 at a fixed angle with each other and the grating is turned. In 

 this case if <p is the angle of deviation and d the angle be- 

 tween the telescopes 



A = — 2 s sin cp cos — 

 n ^2 



These methods are general and the choice between them is 

 simply a question of the convenient application of the appa- 

 ratus at hand. Probably the first and the second methods are 

 the most generally useful, while the third is the most objec- 

 tionable. The method of minimum deviation slightly increases 

 the experimental difficulties, but often improves the defini- 

 tion of the gratings and is capable of giving very exact re- 

 sults. The last method is applicable only when the spectrom- 

 eter is so rigid as to ensure the permanence of the angle 

 between the telescopes. When this condition is fulfilled, how- 

 ever, the method is very valuable, since it reduces the moving 

 mass to a minimum and allows the method of repetition to be 

 readily used. 



In the present research for the work with glass gratings the 

 second method was selected as best suited to the arrangement 

 of the spectrometer. This was a very good instrument by 

 Meyerstein. The circle is 32 cm in diameter, divided on silver 

 to 6' and reading by two microscopes directly to 2" and by 

 estimation easily to within 1". The collimating and observing 

 telescopes are of 4 cm clear aperture and about 35 0m focal 

 length, well corrected and firmly supported. 



For the second part of the work, with speculum metal 



