14 PROCEEDINGS OF THE AMERICAN ACADEMY. 



lamp proved fairly easy to work with in point of steadiness and the 

 average variation of a single deflection from the mean was still less 

 than 1 %. 



Acctylnie Flame. — Following the trial of the Graetzin lamp a 

 series of measurements was made on an acetylene flame fed from a 

 Prestolite tank. This flame gave on the photometer in the direction 

 of measurement 27.35 c. p. and its change in deflection on interposi- 

 tion of the Euphos was .524 cm., corresponding to a cut off of 4.5% 

 of the total energy. It proved very amenable to measurements and 

 was quite as steady and easy to work with as the mantle burner pre- 

 viously used. The spectrum of the acet^dene flame reaches well 

 down into the ultra violet as shown by Schanz and Stockhausen.^^ 

 It reaches, in fact, approximately wave length 310 ;Uyu, but the further 

 portion of the spectrum is comparatively weak. The spectrum of 

 the Welsbach mantle with a clear globe, given by the same authorities 

 (loc. cit.), is cut off at about wave length 320 (jljul, but is notably bright 

 in the part of the ultra violet toward the visible spectrum. These 

 results are fully checked by the spectrograms taken of the particular 

 burners here indicated. 



The Carbon Electric Arc. — Next in order the various arc lamps 

 were taken up for investigation, beginning with the arc between 

 carbon electrodes. On account of the relative instability of the arcs 

 the method of experimentation was modified. A second quartz cell 

 similar to the one already in use was constructed and filled with 

 distilled water. The ratio of the absorption between this new cell 

 and the old cell was then determined. From a slight difference in 

 thickness or in polish of the quartz plates the new cell was found to 

 give about 1% more absorption than the original quartz cell and a 

 correction for this difference was introduced in the subsequent meas- 

 urements. The two quartz cells were mounted in recesses in a sliding 

 screen so that either could be brought quickly in front of the thermo- 

 pile window. The Euphos glass screen was then mounted with a 

 glycerine film on one of the quartz cells so that the cells with and 

 without the Euphos could be rapidly interchanged in the beam from 

 the lamp under test and the absorption thus determined without 

 having to depend on the constancy of the lamp for any considerable 

 time. 



The times of observation were regulated by means of a stop watch 

 so that a time correction for shift of zero could be readily made, and 



'3 Zts. f. Augenheilk., V. XXXIII, plate 8. 



