160 Profs. Hagen and Rubens on some Relations 
between 1 and 8 yw, and of sylvine for the interval between 
8 and 14. ‘The galvanometer used with the thermopile 
was an iron-clad instrument of du Bois-Rubens’ construction*, 
well protected against magnetic disturbances. The observer 
at the galvanometer could, by aid of a special arrangement, 
lift or drop the screen G, placed in front of the slit H. 
In addition to this, the figure shows two supplementary 
mirrors, EH and F, the source of light A, and the concave 
mirror § the reflecting-power of which was to be examined. 

choke S 
Our source of light was a Nernst incandescent lamp, the 
fibre of which (1 mm. thick and 2 mms. wide) was attached 
to a small turning-table in such a way as to permit a kind of 
excentric rotation. This table, as shown in the figure, was 
placed between the mirrors S and E. It was furnished with 
an arm which could be brought into contact with either of 
the two fixed screws C and D, thus permitting the transfer- 
ence of the Nernst fibre from position A into position B 
(dotted in fig. 1), and vice versa. Immediately in front of 
the fibre was adjusted a small slit (2 mms. wide), containing 
two crosses of very thin platinum wire 8 mms. above each 
other. The excentricity of the Nernst fibre and the position 
of the slit in front of it were adjusted by two different 
micrometric screws. 
After undergoing reflexions from the plane mirror E and 
the concave mirror F, the rays coming from A were united 
on the slit H to an image of exactly the same dimensions as 
that produced directly by the source of light. The mirror 8 
* H. du Bois & H. Rubens, Zertschrift fiir Instrumentenkunde, xx. 
p- 65 (1900). 

