234 Royal Society : — 



of measurement. Two strips of this paper are exposed as usual in 

 the pendulum photometer : one of these strips is fixed in hyposul- 

 phite-of-sodium solution, washed, dried, and pasted upon a board 

 furnished with a millimetre-scale. This fixed strip is now graduated 

 in terms of the unfixed pendulum strip by reading off, by the light 

 of a soda-flame, the position of those points on each strip which 

 possess equal degrees of tint, the position of the standard tint upon 

 the unfixed strip being ascertained for the purpose of the gradu- 

 ation. Upon this comparison with the unfixed pendulum strip 

 depends the subsequent use of the fixed strip. A detailed descrip- 

 tion of the methods of preparing and graduating the strips, and of 

 the apparatus for exposure and reading, is next given. The following 

 conditions must be fulfilled in order that the method may be adopted 

 as a trustworthy mode of measuring the chemical action of light : — 

 1st. The tint of the standard strips fixed in hyposulphite must 

 remain perfectly unalterable during a considerable length of time. 

 2nd. The tints upon these fixed strips must shade regularly into 

 each other, so as to render possible an accurate comparison with, 

 and graduation in terms of, the unfixed pendulum strips. 

 3rd. Simultaneous measurements made with different strips thus 

 graduated must show close agreement amongst themselves, and 

 they must give the same results as determinations made by 

 means of the pendulum photometer, according to the method 

 described in the last memoir. 

 The fixed strips are prepared in the pendulum apparatus, and 

 afterwards fixed in hyposulphite of sodium. A series of experi- 

 ments is next detailed, carried out for the purpose of ascertaining 

 whether these fixed strips undergo any alteration by exposure to 

 light, or when preserved in the dark. Two consecutive strips were cut 

 off from a large number of different sheets, and the point upon each 

 at which the shade was equal to that of the standard tint was deter- 

 mined. One half of these strips were carefully preserved in the dark, 

 the other half exposed to direct and diffuse sunlight for periods vary- 

 ing from fourteen days to six months, and the position of equality 

 of tint with the standard tint from time to time determined. It 

 appears, from a large number of such comparisons, that in almost all 

 cases an irregular, and in some cases a rapid fading takes place 

 immediately after the strips have been prepared, and that this fading 

 continues for about six to eight weeks from the date of the prepara- 

 tion. It was, however, found that, after this length of time has 

 elapsed, neither exposure to sunlight nor preservation in the dark 

 produces the slightest change of tint, and that, for many months from 

 this time, the tint of the strips may be considered as perfectly 

 unalterable. 



The value of the proposed method of measurement entirely 

 depends upon the possibility of accurately determining the intensi- 

 ties of the various shades of the fixed strip in terms of the known 

 intensities of the standard strips prepared in the pendulum photo- 

 meter. The author examines this question at length, and details two 

 methods of graduating the fixed strips, giving the results obtained in 

 several series of determinations, in order that the amount of experi- 



