i6 



SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 68 



salt plate fastened on air-tight with white wax. The transmissi- 

 bility of radiation through this plate will be found in table I under 

 the section relating to the prisms. 



The galvanometer is the same as that described on page 32 of 

 Vol. 2 of the Annals of this Observatory. The proportionality of 

 its deflections to the current was tested each year and found to be 

 satisfactory. A time of single swing of about 3 seconds was generally 

 used. The sensitiveness of the apparatus was such that a change of 



^BATTERY,2V0LTS 



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9-50HIVIS,0.13AMPERE5. 



Fig. 5. — Bolometer circuits. 



resistance in R (see fig. 5) of one ohm caused a deflection of 

 nearly 3 cm. 



The galvanometer deflections were recorded on a photographic 

 plate moved by the same clock which moved the spectrum over the 

 bolometer. The following were the various speeds used for the plate 

 and spectrum : 



1908, I m. of time = 4 cm. of plate = 20' of spectrum 

 1914, I m. of time = i cm. of plate = 8' of spectrum 

 1916, I m. of time = i cm. of plate = 4' of spectrum 



