4: PROCEEDINGS OF THE AMERICAN ACADEMY. 



absorption continues slightly on into the violet, gradually fading away 

 until the transmission becomes nearly complete for the bright blue 

 mercury line (435 h/jl). 



In examining b, c and d of Plate 1 it must be remembered that the 

 second order ultra violet overlaps the first order so that the group 

 near 365 /jifj, appears in the first order at the extreme right of the figure 

 and in the second order at the extreme left. In d of this Plate the 

 arc spectrum fades off on the left, not from absorption but from the 

 weakening of the photographic action. The Euphos glass is ex- 

 tremely transparent to the radiations throughout all except the ex- 

 treme violet of the visible spectrum, and well into the infra red, as 

 will hereafter be seen. The results here obtained for its absorption 

 of the ultra violet are altogether parallel with those shown in the 

 paper by Schanz and Stockhausen ^ and also by Hallauer.^ The 

 Euphos glass thus enables a particularly clean partition of the visible 

 spectrum from the ultra violet and extreme violet to be made. 



If it were possible to obtain an equally good absorbent for separat- 

 ing the infra red from the visible spectrum radiometric measurements 

 of efficiency would be greatly facilitated. It should here be noted 

 that Euphos glass appears in various shades and some imitations of 

 it are now upon the market, so that a sample of such glass should be 

 tested in the spectrograph before use for such a purpose as the present, 

 inasmuch as in some of the shades the cut-off of the ultra violet is 

 much less sharp and complete. The sample here used was the original 

 No. 1, 2 mm. thick. 



Method of Investigation. — The method taken for the evaluation 

 was the familiar one of measuring the radiation directly by means of a 

 thermopile connected with a sensitive galvanometer in a manner 

 familiar in recent experiments on the efficiency of illuminants in the 

 visible spectrum, e. g., Lux,^ Fery.^ The thermopile was chosen as 

 the radiometric instrument merely as a matter of convenience. The 

 instrument actually used was a Rubens linear thermopile, having 20 

 constantin-iron couples with a total resistance of 4.6 ohms. It was 

 mounted as shown in Figure 1, in a vacuum tube with a quartz window 

 immediately in front of the couples. The inner body of the instru- 

 ment, containing the couples, was taken out of its original mounting 

 and set up in a tube about 37 mm. in diameter, through the upper 

 end of which was sealed a pair of leading-in wires. 



5 Zts. f . Augpnheilk., May 1910, Table VII, figure .3. 

 ' Archiv. of Ophthal.. Jan. 1910, Plate II, figure 3. 

 « Zts. f. Beleuchtungswesen, Heft 16, 1 p. 36, 1907. 

 ' Bull. Soc. Franc, de Physique, p. 148, 1908. 



