unrecognized Wave-lengths. 



161 



Grating, Rowland No. 1 (concave). 



Prism, HNo. 2 (glass). 



Refracting angle = 62° 15' 03". 



Spectrum thrown east. 



Galvanometer, No. 3, damping magnet at 40 centim. 



Time of single vibration = 21 sec. (with current passing). 



Bolometer, No. 16 (aperture = 1 millim.). 



Reading on slit (before mounting prism), 0° 00' 00". 

 „ „ D 2 (through prism), 52° 52' 40". 



Current of battery = 0"036 ampere*. 



Reader at circle, F. W. V. 



„ galvanometer, S. P. L. 



Object = measurement of deviation of \ = 4x\D 2 in the 

 spectrum of the flint-glass prism H No. 2, with sunlight. 



Galvanometer-deflection with arms in line (from the com- 

 bined effect of all the spectra falling on slit 2), a little 

 over 300 div. 





49 00 



48 58 



48 56 



48 54 



48 52' 



4S firi 







Galvanometer-deflections. 



11 

 2 



10 

 13 



11 

 12 



18 

 15 



19 

 13 



12 

 15 







65 



11-5 



11-5 



16-5 



16 



135 





Galvanometer-deflections. 



13 

 14 



21 



22 



26 

 24 



32 

 31 



25 

 30 



20 

 13 







13-5 



21-5 



25 



31-5 



27-5 



165 





Concluded maximum at 48° 54'. 



Making the galvanometer-deflections ordinates, and the 

 prismatic deviations abscissas, a smooth curve through the 

 points of observation gives in the first case a maximum at 

 48° 54'. The image of the slit has a certain size, nnd so has 

 the bolometer-strip. The latter feels the heat before the 

 centre of strip and image coincide ; and it is this poiut of the 

 coincidence of centres which gives the maximum as denoted 

 by the above figures. 



* It must be understood that this is the total current of the battery. 

 The differential current which passes through the galvanometer is of an 

 altogether different order of magnitude, in this case probably not ex- 

 ceeding 0-000,000,01 ampere. 



Phil Mag. S. 5. Vol. 22. No. 135. August 1886. M 



