Royal Society. 233 



is Jacobi's imaginary transformation in the Theory of Elliptic 

 Functions. See, as to this, my paper " On the Transcendent 



gd.w=-vlogtan/J+-MiY" Phil. Mag. vol. xxiv. (1862) 

 pp. 19-22. 



Cambridge, January 21, 1865. 



XXXIV. Proceedings of Learned Societies. 



ROYAL SOCIETY. 



[Continued from p. 157.] 

 December 22, 1864. — Dr. William Allen Miller, Treasurer and Vice- 

 President, in the Chair. 

 npHE following communication was read: — 

 X " On a Method of Meteorological Registration of the Chemical 

 Action of Total Daylight." By Henry E. Roscoe, B.A., F.R.S. 



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 Photochemical 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 sensitive- 

 ness — light of the intensity 50, acting for the time 1, thus produ- 

 cing 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 pen- 

 dulum are known. The strip of sensitive paper insolated during the 

 oscillation of the pendulum exhibits throughout its length a regu- 

 larly 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 sepa- 

 rate determinations can be made upon 36 square centimetres of sen- 

 sitive paper. Strips of the standard chloride-of-silver paper tinted 

 in the pendulum photometer remain as the basis of the new mode 

 * Phil. Trans. 1863, p. 139. 

 Phil. May. S. 4. Vol. 29. No. 195. March 1865. R 



