38 LIGHT IN RELATION TO TREE GROWTH. 
changed when exposed to the light, the shades obtained by different 
exposures of the “ normal paper ” are reproduced in permanent water 
colors. The different standards are then placed in the insolator 
beside the “ normal paper” for facilitating comparison. (See fig. 6, 
No S210 
In field work, instead of the “normal paper,” ordinary photo- 
graphic paper, especially Rodamin-B paper, invented by A. An- 
dresen, of Berlin, may be used. The chief drawback to the use of 
the “normal paper ” is its poor keeping quality. It can be kept at 
most only 24 hours, and therefore a fresh supply must be made every 
day. Any commercial silver chloride or silver bromide photographic 
papers may be used, provided they are of a uniform known sensi- 
tiveness with reference to the 
, in | ‘normal paper.” 
CLEMENTS’ PHOTOMETER. 
Prof. F. E. Clements (1905: 
38-63), in his studies of the 
infiuence of ight upon vege- 
tation, has used a photometer 
of his own invention which 
in many ways is simpler than 
Wiesner’s insolator. Like 
Wiesner’s photometer, it is 
also based on the blackening 
of silver salts in the light. 
The construction of the in- 
ers: strument is shown in fig. 7. 
pe bg ee It is made in two cases, outer 
= QQ A ])Nywwv SN vilw ae pate aes - 
Me 6 wiesnetavincolatons Sit in athe tight metal case. A strip of 
black paper; N. P., “normal paper”; N. S. sensitive paper is fastened 
1 pormal singe” (t'unit); 8: 8 29, upon the shoulder of B, which 
fits into the outer case A. 
The two parts are held together by means of a central screw C. In 
the outer case is a hole one-fourth of an inch square, which is opened 
and closed by means of a slide S working between two flanges (/). 
By turning the inner case B the strip of paper is made to revolve 
past the opening. The outer face of B is graduated into 25 equal 
parts, which are numbered consecutively in such a manner that the 
number opposite the opening will always indicate the number of the 
exposure. On the edge of the shoulder bearing the strip of paper 
are 25 holes, which are engaged by a spring catch as the case B 
is turned, thus allowing only one twenty-fifth of a revolution to be 
