608 
PROFESSOR ROSCOE ON A METHOD OF METEOROLOGICAL 
The following conditions must be fulfilled in order that this method can be adopted 
as a reliable measurement of the chemical action of light : — 
1st. The tint of the standard strips fixed in hyposulphite must remain perfectly 
unalterable during a considerable length of time. 
2nd. The tints upon these fixed strips must shade regularly into each other, so as to 
render possible an accurate comparison with, and graduation in terms of, the 
unfixed pendulum strips. 
3rd. Simultaneous measurements made with different strips thus graduated must 
show close agreement amongst themselves, and they must give the same results 
as determinations made by means of the pendulum photometer, according to 
the method described on pages 158, 159 of the last memoir. 
I. Preparation of the Standard fixed Strips. 
For the purpose of preparing the fixed strips, sheets of good white photographic 
paper are salted in a solution containing 3 per cent, of chloride of sodium, exactly 
according to the directions given in the last memoir (p. 155) for the preparation of the 
standard paper. The salted paper after drying is cut into pieces, 16 centimetres in 
length by 15 centimetres in breadth, and silvered on a bath containing 12 parts of 
nitrate of silver to 100 parts of water. After drying, one of these papers is fixed at the 
corners upon a board covered by a well-fitting lid of sheet zinc, so made that it does not 
touch the paper ; the paper is then blackened by exposure to the action of light in the 
pendulum apparatus. For this purpose, the thin elastic sheet of the blackened mica 
usually employed, is replaced by a piece of thin sheet zinc 16 centimetres broad. The 
frame carrying the paper is clamped on to the horizontal plate of pendulum photo- 
meter, and the sheet of blackened zinc placed over it ; the cover is then withdrawn, 
and the paper exposed by allowing the pendulum, with the sheet of zinc attached to it, 
to vibrate until the required tint has been attained. The cover is then replaced, the 
frame opened in the dark room, the paper washed to remove excess of nitrate of silver, 
fixed in a saturated solution of hyposulphite of sodium, and well washed for three days. 
As the tints of the foxy-red colour which the paper possesses after fixing can be accu- 
rately compared with the bluish-grey tint of the freshly-exposed paper by means of the 
monochromatic light of the soda-flame, the use of a toning-bath was specially avoided 
as likely to render the paper liable to fade. Each sheet thus prepared is cut into four 
strips, 160millims. long and 30 millims. broad, which are then preserved for graduation. 
In order to ascertain whether these fixed strips undergo any alteration in tint by 
exposure to light, or when preserved in the dark, two consecutive strips were cut off 
from several different sheets, and the point on each at which the shade was equal to that 
of the standard tint (see last memoir, p. 157) was determined by reading off with the 
light of the soda-flame, by means of the arrangements fully described on p. 143 of the 
above-cited memoir. One-half of these strips were carefully preserved in the dark, the 
other half exposed to direct and diffuse sunlight for periods varying from fourteen days 
to six months, and the position of equality of tint with the standard tint from time to 
