8 PROCEEDINGS OF THE AMERICAN ACADEMY. 



As a check on the possible magnitude of this virtual absorption by 

 the glycerine film readings were taken on a tungsten lamp through 

 the quartz cell alone, and through the quartz cell plus a disc of optical 

 crown glass 2 mm. thick secured with glycerine in the ordinary man- 

 ner. The absorption of this crown glass is shown in Plate 1, e, f, g, in 

 which e is the spectrogram of the quartz arc taken with a wide slit 

 and 2 minutes exposure, / the spectrogram through the crown glass in 

 question, and g through the Euphos glass. In spite of the fact that 

 there is a slight absorption by the crown glass in the region near 

 300 iJLiJL, the addition of the crown glass and glycerine film reduced the 

 galvanometer deflection by barely 0.5 %, an amount scarcely out- 

 side the errors of observation. The energy cut off from the spectrum 

 of a tungsten lamp by the crown glass would be of course very 

 small, but perhaps not negligible, since as Schanz and Stockhausen 

 have shown (loc. cit. table VIII, figure 6) the tungsten lamp spectrum 

 goes quite down to 300 /x)u in sufficient strength to give a clear photo- 

 graphic effect. At all events it is evident that the use of the glycerine 

 film involves no material errors. 



In the ordinary experimentation in using steady sources, sets of 

 readings were taken alternately with and without the Euphos glass, 

 the glass being either added to the clear cell with the glycerine film, 

 or removed and the film quickly washed away with distilled water. 

 With sources which give trouble from unsteadiness the second quartz 

 cell was brought into play as previously mentioned. Aside from a 

 slight drifting of the zero point, which is generally observable in 

 measurements with a thermopile, the method adopted worked very 

 smoothly. The drift, however, was usually small and slow and satis- 

 factorily taken care of by a time correction. With proper attention 

 to this, the readings, although necessarily slow, were nearly as consis- 

 tent as would be found in ordinary photometric measurements. 

 The following string of deflections forming a single group of 5 readings 

 is typical of those obtained under ordinary conditions. 



Scale readings from bare quartz lamp through quartz cell only. 



cm. 



36.17 

 36.10 

 36.27 

 36.36 

 36.16 



Av. =36.21 



