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XVII. On the Chemical Intensity of Total Daylight at Kew and Para, 1865, 1866, 
and 1867. By Henry E. Roscoe, F.B.S. 
Received May 14, — Read June 20, 1867. 
Part I.— THE KEW OBSERVATIONS. 
In the year 1864 1 communicated to the Royal Society* the description of a method for 
the Meteorological Registration of the Chemical Intensity of Total Daylight, founded 
upon an exact measurement of the tint which standard sensitive paper assumes when 
exposed for a given time to the action of daylight. 
During the last two years measurements of the chemical intensity, according to this 
plan, have, through the kindness of Dr. Balfour Stewart, been made regularly every 
day at the Kew Observatory by Mr. T. W. Baker, and thus the practicability of carrying 
out a continued series of observations according to this method has been effectually and 
satisfactorily tested. 
Owing to the press of regular work at the observatory only three separate registra- 
tions of chemical intensity could be made at different hours each day. Hence the 
results obtained do not in any way indicate the hourly variation of chemical intensity, 
nor can even the individual integrals of daily intensity, giving the mean chemical action 
each day, be said to do more than exhibit approximately the changes which go on from 
day to day. The monthly integrals, on the other hand, each calculated from a large 
number of observations, show with a great degree of accuracy the rise and fall of the 
chemical intensity with the changing seasons of the year, and enable us to deduce from 
this the first series of observations of the kind, the mean monthly and yearly chemical 
intensities at Kew for 1865, 1866, and 1867. 
The hours at which the chemical intensity was registered each day were those chosen 
for the reading of the meteorological instruments, viz. 9 h SO" 1 a.m., 2 h 30 m p.m., and 
4 h 30 m p.m. The condition of the sun’s surface as regards freedom from cloud, the 
amount of the cloud, the temperature (wet and dry bulb) and the atmospheric pressure 
were also noted. 
As an example of the results thus obtained, the observations made in the month of 
July 1866 have been chosen as exhibiting well the great changes in chemical intensity 
produced by varying cloud and sunshine. 
* Bakerian Lecture, Philosophical Transactions, 1865, Part II. p. 605. 
4 G 
MDCCCLXVII. 
