350 L. Bell — Absolute Wave-length of Light 



The method of determining <p was as follows : The instru- 

 ment being adjusted by the ordinary methods, a suitable line 

 was selected for measurement and then the angle 6 was slightly 

 increased or diminished until by measurement of a double de- 

 flection n<p was found to be very close indeed to 360°. Then 

 a double deflection was carefully measured and if time per- 

 mitted several times repeated, an observer always being at the 

 eye-piece to see that the line should not move from the cross 

 hairs while the micrometers were being read. Then, clamping 

 the main circle, the grating holder was turned through 2(p until 

 the line was very closely upon the cross hairs, any slight read- 

 justments made necessary by this disturbance of the instrument 

 were made, and the process was repeated. In this way the ini- 

 tial line of the circle was finally reached and a value of nip ob- 

 tained which depended only on the algebraical sum of the 

 micrometer readings, always a small quantity. 



The determination of the temperature, a very difficult and 

 uncertain matter in the case of glass gratings, is here compara- 

 tively simple. A sensitive thermometer (Baudin 6156) was kept 

 in contact with the grating, its bulb being carefully shielded 

 by cotton. The construction of the spectrometer made it im- 

 practicable effectively to shield the grating from radiation from 

 the observer's body ; but the thermometer apparently proved 

 effective in giving the real temperature since no discrepancies 

 in the results could be traced to thermal causes. The thermom- 

 eter readings were made to o, 05, and the temperature of obser- 

 vation rarely varied more than two or three degrees from 20° C. 



The temperature being thus obtained, the necessary correction 

 was introduced directly into the angle of deviation. Writing 

 the formula for wave-length in the form 



A = C s sin cp, 



where C is a factor depending on the method in which the 

 grating is used, and differentiating we obtain 



— = — cot q>dq), 



s 



ds 

 where if we take 1° for the temperature variation — is the co- 

 efficient of expansion. Whence 



ds 

 s 



Scp= — 



cot <p 



correction fori variation in temperature. For grating III for 

 instance <?^=2 //- 688 and by this means all the deviations were 



