346 M. H. C. Vogel on the Absorption of the 



determined. I have endeavoured to solve this question, and 

 take the liberty to communicate now my observations. 



Herschel*, Claudetf, and others have already tried to deter- 

 mine the intensity of light by photographic colouring ; but the 

 experiments did not succeed in consequence of the difficulty of pre- 

 paring a photographic layer of the same sensitiveness throughout, 

 and of discovering a law connecting the time of exposure, intensity 

 of light, and the shade of tint of the photographic paper. The 

 photochemical researches of Bunsen and RoscoeJ have even 

 shown that no proportionality exists between the intensity of the 

 light and the tint of the photographic paper after a certain 

 time of insolation. Their observations, which were made with 

 great care, showed " that equal products of the intensity of the 

 light into the times of insolation correspond within very wide limits 

 to equal shades of tints produced on chloride-of-silver paper of 

 uniform sensitiveness. }> Calling the intensity of light I, and the 

 time of insolation t, the equation It = I l t 1 exists for equal shades. 

 By the discovery of this important law we are enabled to express 

 the intensity of light in comparable measures. 



The observations of Bunsen and Roscoe only relate to the 

 chloride-of-silver paper, but do not remain valuable for photo- 

 graphs on collodion, since the development of the image on collo- 

 dion changes the relative intensity of the shades, as is known to 

 every photographer. I have therefore prepared photographs of 

 the sun on chloride-of-silver paper in order to measure the in- 

 tensity of the light emitted by different points of the sun's disk. 



The paper was carefully prepared, stretched over a plate of 

 glass, and exposed in the camera fixed to the telescope. Two 

 images of the sun were prepared on the 8th of March, 1872 ; 

 one was exposed during 30 seconds, the other during 40 

 seconds. The diameter of the images was 108 millims. ; the 

 weakening of the light towards the edge of the disk was clearly 

 seen in both. I then determined four points of equal intensity 

 by a scale prepared photographically §. We assume that the 

 intensity i of the light used for the preparation of the scale was 

 constant, and that the time t of exposure was the same for all 

 points of the sun's image. If I is the intensity of the light in 

 the middle of the sun's disk, I x that at another point, we have 



l t=it lt 



* Phil. Trans. 1840, p. 46. f Phil. Mag. vol. xxxiii. p. 339. 



X Pogg. Ann. vol. cxvii. p. 329.- 



§ I think it superfluous to give a detailed description of the manner in 

 which the scale was prepared and the comparison with the sun's image 

 made, as the method was in general the same as tbat used by Bunsen and 

 Roscoe, and described by them accurately in their ' Photochemical Re- 

 searches/ 



