1910-11.] Absorption of Light by Inorganic Saits. No. IV. 551 
their extreme positions, namely, 20 and 10 cm. respectively, the illumina- 
tion of B 1 is J that of B 2 . In order to increase the effective range of the 
photometer a piece of wire mesh E is placed between the arc G and the 
quartz plate B r Several pieces of mesh were treated with nitric acid for 
different lengths of time, and that one was chosen which allowed about 
one-quarter of the incident light to pass through it. Thus the intensities 
of illumination of Bj and B 2 are the same when <^ = 10 cm. and d 2 = 20 
cm., and the illumination of B 2 is sixteen times that of Bj when d 1 — 20 cm. 
and d 2 = 10 cm. 
At first, instead of using the mesh, a screen was pushed into the upper 
beam from one side at a point close up to the prism, in order to diminish 
the intensity. It displaced the lines in the spectra, however, and thus did 
not prove satisfactory. 
The essential parts of the photometer are shown in position in the 
above diagram. B x and B 2 are the two square plates of ground quartz ; 
they rest in metal supports fixed to upright wooden stands, which can be 
moved backwards and forwards alongside the horizontal scale L. The 
metal supports are so made as to serve the additional purpose of screening 
light off all but the necessary portion of each of the quartz plates. C is the 
quartz prism of special design, enclosed in a metal frame. D is an upright 
wooden screen with a rectangular slot in it, and E is the wire mesh nailed 
across the upper half of the slot. A zinc screen F is placed in front of the 
iron arc G which is formed by two horizontal converging rods passing 
through a plaster of Paris mould. H is a quartz cell for holding the 
solution to be examined. All the woodwork connected with the photometer 
and all the metal supports were painted a dead black. 
Imperial Special Rapid plates were used. With exposures of about £ 
min. and a slit width of about 1 mm. a fairly continuous spectrum extend- 
ing to A = *233/^ was obtained with the iron arc as source, and with slightly 
