1038 



order to obtain a greater intensity of ligiit. Moreover a potassinna 

 bichromate filter M was placed in front of F by means of which 



-^ 



Fig. ]. 



in the comparison of furnace and gi-ound-glass the colour could be 

 matched. 



The measurement was made as follows. The plate 1 with the 

 prisms was removed and the temperature of the furnace was checked 

 until it was properly constant. / was then replaced and with the 

 sector L in motion the brightness of the ground-glass F as seen 

 through the telescope A was made equal to that of the alundum 

 disc in the furnace. This was attained by varying the current in the 

 projection-lamp. As soon as equality was obtained the prisms BCD 

 were once more removed ; the furnace-temperature was then again 

 determined and the ground-glass F was measured photometrically 

 with the sector open. In this measurement the intensity was ^ 

 times greater than in the comparison furnace-ground glass. The photo- 

 metric measurement of F with the bichromate filter in front was 



