406 Scientifie Intelligence. 
by powdered silver chloride, chemically pure,—which was found 
to be much more sensitive than that made from the commercial 
chemicals. This powder is spread out in a thin layer, in a long 
paper cell, on a strip of glass. The cell measures one centimeter 
broad by ten in length. Over this is laid a sheet of tissue paper 
and above that a narrow strip of black paper, so arranged as to 
little over a meter in length, closed at the top by a board, in 
which is a circular aperture 15°8 cm. in diameter. Over this 
tissue paper, of varying widths, and arranged like a flight of 
: 
unity, the strips on either side taken in order will transmit 
approximately,— 
1 2 3 4 5 6 7 8 9 
PO 266. 78 1°2 1:0 "85 a 6 5 
and two and a half in width, are now placed side by side under 
the scale, in the place of the chloride. By this means we can test, 
oO 
the sensitometer containing the -05 cm. hole is put on, and the 
§ again periormed as soon as the plates have been removed, and 
the first result confirmed, With some plates it is necessary © 
make two or three trials before the right exposure can be found, 
