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XII. The Bakerian Lecture. — On a Method of Meteorological Registration of the 
Chemical Action of Total Daylight*. By Henry Enfield Roscoe, B.A., F.B.S . , 
Professor of Chemistry in Owens College , Manchester. 
Received November 8, — Read December 22, 1864. 
In the last memoir on Photochemical Measurements, presented to the Royal Society f, 
Professor Bunsen and I described a method for determining, by simple observations, the 
varying amount of chemical action effected by the direct and diffuse sunlight on photo- 
graphic paper, founded upon a law discovered by us, viz. that equal products of the 
intensity of the light into the times of insolation correspond within very wide limits to 
equal shades of tints produced on chloride-of-silver paper of uniform sensitiveness — so 
that light of the intensity 50, acting for the time 1, produces the same blackening effect 
as light of the intensity 1 acting for the time 50. For the purpose of exposing this paper 
to light for a known but very short length of time, a pendulum photometer was con- 
structed ; and by means of this instrument a strip of paper is so exposed that the different 
times of insolation for all points along the length of the strip can be calculated to within 
small fractions of a second, when the duration and amplitude of vibration of the pen- 
dulum are known. The strip of sensitive paper insolated during the oscillation of the 
pendulum exhibits throughout its length a regularly diminishing shade from dark to 
white ; and by reference to a Table, the time needed to produce any one of these shades 
can be ascertained. The unit of photo-chemical intensity is assumed to be that of the 
light which produces upon the standard paper in the unit of time (one second) a given but 
arbitrary degree of shade termed the normal tint. The reciprocals of the times during 
which the points on the strip have to be exposed in order to attain the normal tint, give 
the intensities of the acting light expressed in terms of the above unit. 
According to this method the chemical action of the total daylight (*. e. the direct 
sunlight and the reflected light from the whole heavens) has been determined, by means 
of observations made at frequent intervals throughout the day, and curves representing 
the variation of daily chemical intensity at Manchester have been drawn The labour 
of obtaining a regular series of such daily measurements of the chemical action of day- 
light according to this method is, however, very considerable ; the apparatus required 
* It is to be carefully borne in mind that no absolute measurement of the more refrangible solar rays falling 
on the earth’s surface is possible, except by the expression of their heat-producing effect ; and that all methods 
of measuring the intensity of these rays depending upon the action which they produce on any single chemical 
compound, give results which are only true for the particular rays affecting the compound selected as the standard 
of comparison. 
t Philosophical Transactions, 1863, p. 139. + Ibid. 1863, p. 160. 
MDCCCLXV. 4 0 
