258 Captain Abney and Major-General Festing. [Dec. 8, 



such a current is passed through the lamp that it emits a medium 

 light. A time scale is then made by giving different lengths of expo- 

 sure at each movement of the plate in its last half. The plate is then 

 taken out of the slide, and the images developed. By this means both 

 a time scale and a measure of intensity for different temperatures are 

 on the same plate in the shape of squares of different density of 

 deposit. "When the negative is dry it is placed in an apparatus which 

 works on the principle of the optical lantern, and is described in the 

 paper written by one of us, " Atmospheric Absorption of Sunlight " 

 (* Phil. Trans.,' 1887), and the density of each square measured. The 

 " intensity " measures are then compared with the time scale, and 

 the value of the intensity calculated. From these values can be deter- 

 mined if the curve of intensity and watts increases parabolically. It 

 might be objected that increase in intensity is not convertible into 

 " time of exposure." Careful experiments have been made as to this, 

 and for the range of time which is comprised in the seconds of 

 exposure given no appreciable error ensues. 



The following is an example of an experiment conducted in the 

 above manner : — 



Table VI. 

 Time Scale. 



No. of aperture. 



Exposure given 

 to portion of 

 plate. 



Light transmitted 

 through de- 

 veloped plate. 





sees. 





1 



5 



55-0 



2 



10 



47-5 



3 



15 



39-5 



4 



20 



33-0 



5 



25 



27'5 



6 



30 



23-2 



7 



35 



20-0 



8 



40 



18*1 



9 



45 



16 -0 



10 



50 



141 



11 



55 



13*2 



12 



60 



12 *4 



Bare glass = 56. 



