556 Prof. Roscoe — Registration of Action of Daylight. [Dec. 22, 



in hyposulphite-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 graduation. Upon this com- 

 parison with the unfixed pendulum strip depends the subsequent use of the 

 fixed strip. A detailed description of the methods of preparing and gra- 

 duating 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 gra- 

 duation in terms of, the unfixed pendulum strips. 



3rd. Simultaneous measurements made with different strips thus gradu- 

 ated must show close agreement amongst themselves, and they must 

 give the same results as determinations made by means of the pendu- 

 lum photometer, according to the method described in the last 

 memoir. 



The fixied strips are prepared in the pendulum apparatus, and after- 

 wards fixed in hyposulphite of sodium. A series of experiments 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 determined. One half of these strips were carefully pre- 

 served in the dark, the other half exposed to direct and diffuse sunlight for 

 periods varying 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 preparation. 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 intensities of the various 

 shades of the fixed strip in terms of the known intensities of the standard 

 strips prepared in the pendulum photometer. The author examines this 

 question at length, and details two methods of graduating the fixed strips. 



