606 
PROFESSOR ROSCOE ON A METHOD OF METEOROLOGICAL 
is bulky, the observations can only be made in calm weather, and the quantity of sensi- 
tive paper needed for a day’s observations is large. 
The aim of the following communication is to describe a very simple mode of deter- 
mining at any moment the chemical action of the whole direct and diffuse sunlight (as 
measured by chloride-of-silver paper) adapted to the purpose of regular meteorological 
registration, and founded upon the principles laid down in the memoir above alluded to. 
According to this method a regular series of daily observations can without difficulty be 
kept up at frequent intervals. The whole apparatus needed for exposure can be packed 
into very small space ; the observations can be carried on without regard to wind or 
weather; and no less than forty-five separate determinations can be made upon 
36 square centimetres of sensitive paper. 
Strips of the standard chloride-of-silver paper tinted in the pendulum photometer 
remain as the basis of the more simple mode of measurement now to be described. 
Two strips of this paper are exposed as usual in the pendulum photometer ; one of these 
strips is fixed in hyposulphite-of-sodium solution, washed, dried, and pasted upon a 
board furnished with a millimetre-scale. This fixed strip is now graduated in terms of 
the unfixed pendulum strip by reading off, with the light of a soda-flame, the position 
of those points on each strip which possess equal degrees of tint, the position of the 
normal tint upon the unfixed strip being ascertained for the purpose of the graduation. 
The fixed strip thus becomes in every respect equivalent to the unfixed strip. Upon 
this comparison with the unfixed pendulum strip depends the subsequent use of the 
fixed strip. In order to understand how the chemical action of daylight can be 
measured by help of this fixed and graduated strip, let us suppose, in the first place, that 
we have ascertained the position of those points upon the fixed strip which possess an 
equal degree of tint to points on the unfixed strip situated at regular intervals, say 
10 millims. from each other. By reference to Table I. of the above-mentioned memoir, 
given below, we then find the relation between the times of exposure necessary to effect 
the tints in question when the intensity of the light remains constant. 
Let us suppose, in the second place, that the position on the unfixed strip of 
which the shade corresponds to that of the normal tint has been found ; and that 
the time of exposure, placed opposite to this position in Table I., has been noticed. 
If, now, the various tints on the strip had been produced in one and the same time by 
lights of different intensities, instead of being effected by light of the same intensity 
acting for different times, the law above alluded to shows that the numbers found in 
the Table would represent the relation of these different intensities ; so that in order 
to express this relation in terms of the unit of intensity employed, it is only necessary 
to multiply the numbers thus obtained by a constant, viz. the reciprocal of the number 
found in column II. of the Table, opposite to the position in column I., giving the point 
on the unfixed strip equal in shade to the normal tint. An example may serve to 
make this calculation plain : (1) The position on the unfixed strip equal in shade to the 
normal tint was found to be 112 millims. ; (2) the positions on the fixed strip equal in 
