564 
PROFESSOR ROSCOE ON THE CHEMICAL INTENSITY OF 
days of April 18th, 23rd, 25th, and 26th. The curves for these days, compared with 
the dotted lines below, indicating the corresponding action at Kew, show the enormous 
variation in chemical intensity which occurs under a tropical sun in the rainy season. 
Regularly every afternoon, and sometimes at other hours of the day, enormous thunder- 
clouds obscure the sun, and discharging their contents in the form of deluging rain, 
reduce the chemical action nearly to zero. The storm quickly passes over and the 
chemical intensity rapidly rises to its normal value. 
If we compare the daily mean intensities at Para and Kew on the same days, we gain 
some idea of the true chemical action of the tropics, and it becomes at once evident that 
the alleged failure of photographers cannot, at any rate, be ascribed to a diminution in 
the sun’s chemical intensity, but must rather he referred either to overexposure of the 
plate, or more probably to the difficulty of obtaining a distinct image owing to constant 
variation in the density of the layers of air intervening between the plate and the object. 
The curves, figs. 9 to 14, Plate XXI., exhibit graphically the relation of chemical 
intensity at Kew and Para on the 18th, 23rd, 24th, 25th, and 26th of April 1866, these 
being chosen from the other sets of observations as being the most complete. The data 
for these five days’ observations are found in the Tables at the end of this paper. 
The following numbers give the Daily Mean Chemical Intensities at Kew and Para 
for fifteen days in April 1866. 
Date. 
Daily mean Intensity. 
Patio. 
Kew. 
Para. 
1866. 
April 4... 
269-4 
6 ... 
28-6 
242-0 
8-46 
7 ... 
7-7 
301-0 
39-09 
9... 
5-9 
326-4 
55-25 
11 ... 
25*4 
233-2 
9-18 
12 ... 
55*8 
203-1 
3-66 
13 ... 
52:2 
337 8 
6-46 
14 ... 
38-5 
265-5 
6-89 
18 ... 
39-8 
350-1 
8-80 
19 ... 
75-2 
352-3 
4-68 
20 ... 
38-9 
385-0 
9-90 
23 ... 
80-4 
350-1 
4-35 
24 ... 
83-6 
362-7 
4-34 
25 ... 
73-7 
307-8 
4-17 
26 ... 
39-1 
261-1 
6-67 
Mean in-\ 
tensity, j 
46-06 
303-2 
Hence it appears that the chemical action of total daylight in the month of April 1866 
was 6 - 58 times as great at Para as at Kew. 
In order to form an idea of the march of the daily chemical intensity under the 
equator in the sunshine, all the observations made when the sun’s disk was unobscured 
by clouds have been collected, and a curve plotted out from the means thus obtained. 
