1864.] Uegistration of the Chemical Action of Daylight, 555 



throw out the factors ^, thus arriving at a result independent of 

 (X, ju, j ); viz. this is the before-mentioned equation of the order 12m— 27. 

 The difficulty of the investigation consists in obtaining the transformations 

 by means of which the equation in its original form is thus divested of these 

 irrelevant factors. 



IV. On a Method of Meteorological Registration of the Chemical 

 Action of Total Daylight.^' By IIenky E. Roscoe, B.A., F.R.S. 

 Received November 8, 1864. 



(Abstract.) 



The aim of the present communication is to describe a simple mode of 

 measuring the chemical action of total daylight, adapted to the purpose of 

 regular meteorological registration. This method is founded upon that 

 described by Prof. Bunsen and the author in their last Memoir* on Photo- 

 chemical Measurements, depending upon the law that equal products of the 

 intensity of the acting light into the times of insolation correspond within 

 very wide limits to equal shades of tints produced upon chloride-of-silver 

 paper of uniform sensitiveness — light of the intensity 50, acting for the 

 time 1, thus producing 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 

 constructed ; 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 pendulum 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 action is 

 assumed to be that intensity of light which produces in the unit of time 

 (one second) a given but arbitrary degree of shade termed the standard 

 tint. The reciprocals of the times during which the points on the strip 

 have to be exposed in order to attain the standard tint, give the intensities 

 of the acting light expressed in terms of the above unit. 



By means of this method a regular series of daily observations can be 

 kept up without difficulty ; the whole apparatus needed can be packed up 

 into 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 new mode of measurement. Two strips of this paper are 

 exposed as usual in the pendulum photometer : one of these strips is fixed 



* Phil. Trans. 1863, p. 139. 



