Intensity of Rays of High Ref Tangibility . 



279 



and subsequently dried by means of sulphuric acid. In 

 order to determine the actinic intensity of the light, a modified 

 form of Bunsen and Roscoe's pendulum-actinometer was 

 used (Trans. Roy. Soc. 1862, p. 139) ; in this method paper 

 coated with silver chloride is exposed to light, and the time 

 required to produce a degree of darkening equal to a standard 

 tint is measured. The intensity of light necessary to pro- 

 duce this tint in one second of time is taken as the unit of 

 intensity. The measurements were made in the open air, 

 care being taken that the air- and chlorine-tubes were ex- 

 posed to light in the same plane as the sensitive paper. 

 When the maximum expansion of the chlorine was reached at 

 any one time, as shown by the index remaining stationary, 

 the intensity of the light was measured by exposing the sen- 

 sitive paper. Two series of observations were made under 

 widely different conditions of light ; one on June 8, which 

 was a cloudy day, the other on June 9, on which the sun was 

 shining. The results of these measurements are given in the 

 following table : — 



First Series. 





I. 



II. 



III. 



IV. 



Time. 



Intensity of light 

 (unit =1). 



Expansion of CI, 

 expressed in centi- 

 metres on scale. 



Eatio * 



Recalculation 

 of intensities 

 from observed 











expansion. 



m. s. 











2 50 



0-0242 



1-95 



0-0124 



0263 



2 55 



0-0571 



3-20 



0-0178 



00432 



3 



0-0461 



320 



00144 



0-0432 



3 5 



00291 



1-87 



0-0155 



0-0252 



3 15 



00196 



1-40 



0-0140 



0-0188 



4 



0-0382 



2-70 



0-0141 



00364 



Second Series. 



10 45 



0-3758 



28-15 



0-0133 



0-3799 



10 50 



01022 



7-95 



0-0128 



0-1073 



11 



0-1693 



12-2 



00138 



0-1647 



11 5 



0-3240 



27-95 



0-0116 



0-3773 



11 15 



0-4381 



26-20 ? 



00167 



0-3537 



11 30 



0-3348 



28-0 



00119 



0-3779 



11 35 



0-3212 



29-95 



0-0107 



0-4042 



11 36 



0-4457 



30-80 



00144 



0-4158 



12 20 



0-0779 



6-15 



00126 



0-0890 



12 30 



0-1210 



7-70 



0-0157 



0-1040 



12 32 



0-1087 



7-65 



0-0142 



01032 



12 35 



01133 



9-25 



00122 



0-1249 



4 



0-1108 



1075 



00103 



01451 



4 5 



0-1141 



1005 



00113 



0-1357 



4 15 



0-1150 



7-75 



0-0148 



0-1046 



