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XVII. On the Measurement of the Chemical Intensity of Total Daylight made at Catania 
during the Total Eclipse of Dec. 22nd, 1870. Dy Henry E. Roscoe, Pro- 
fessor of Chemistry, Owens College, Manchester , and T. E. Thorpe, F.R.S.E. , Professor 
of Chemistry, Andersonian University, Glasgow. 
Received June 15, — Read June 15, 1871. 
The following communication contains the results of a series of measurements of photo- 
chemical action made at Catania in Sicily, on Dec. 22nd, 1870, during the total solar 
eclipse of that date, with the primary object of determining experimentally the relation 
existing between this action and the changes of area in the exposed portion of the sun’s 
disk. The attempt to establish this relation has already been made by one of us from 
the results of observations carried out by Captain John Herschel, R.E., F.R.S., at 
Jamkhandi, in India, during the total eclipse of August 18th, 1868*. Unfortunately 
the weather at Jamkhandi at the time of the eclipse was very unfavourable for observa- 
tion ; the estimated amount of cloud during the time of the eclipse amounted to about 7, 
the sun occasionally being even completely obscured. 
In addition to the errors arising from the unsettled state of the weather, a further 
element of uncertainty was unavoidably introduced in the subsequent calculation in 
allowing for the variation in chemical intensity caused by the alteration in the sun’s 
altitude during the progress of the eclipse. It has been shown that the relation between 
the sun’s altitude and the chemical intensity at any given place is represented by the 
equation 
CI tt = CI 0 + const. X u, 
where CI a signifies the chemical intensity at any altitude (a) in circular measure, Cl 0 
the chemical intensity at 0°, and const, a a number derived from the observations f. Since 
no special series of observations were made at Jamkhandi (lat. 16° 30' N.) in order to 
determine the constant, its approximate value could only be obtained from observations 
made at Para, in Brazil (lat. 1° 25' S.), during a different season of the year. 
It appeared from these observations that, as might be expected, the rate of diminu- 
tion of the chemical intensity of total daylight during the first portion of the eclipse up 
to the point at which the disk is half obscured is greater than corresponds to the area 
of darkened solar disk, whilst from this point up to totality the rate of diminution of 
chemical action is much less than that of the exposed portion of the disk. 
The occasion of the recent solar eclipse presented a favourable opportunity for rede- 
* Roscoe, Mem. Lit. and Phil. Soc. of Manchester, 1868-69, vol. iv. [3] p. 202. 
t Philosophical Transactions, 1867, p. 555 ; 1870, p. 315. 
