British Association for the Advancement of Science. 97 



hoped to obtain a numerical estimate of the intensity of solar 

 light at different periods of day, and in different parts of the globe. 

 It consisted of a sheet of photogenic paper, moderately sensible, 

 rolled round a cylinder, which by means of machinery, would 

 uncoil at a given rate, so as to expose to the direct action of the 

 solar rays, for the space of an hour, a strip of the whole length of 

 the sheet, and of about an inch in diameter. Between the paper 

 and the light was to be interposed a vessel, with plane surfaces of 

 glass at top and bottom, and in breadth corresponding to that of 

 the strip of paper presented. This vessel, being wedge-shaped, 

 was fitted to contain a body of fluid of gradually increasing thick- 

 ness, so that, if calculated to absorb light, the proportion intercep- 

 ted, would augment in gradually increasing proportion from one 

 extremity of the vessel to the other. Hence it was presumed 

 that the discoloration arising from the action of light would pro- 

 ceed along the surface of the paper to a greater or less extent, 

 according as the intensity of the sun's light was such as enabled 

 it to penetrate through a greater or less thickness of the fluid 

 employed. In order to register the results, nothing more was 

 required than to measure, each evening, by means of a scale, how 

 many degrees the discoloration had proceeded along the surface 

 of the paper exposed to light, during each successive hour of the 

 preceding day. To render the instrument self-registering, some 

 contrivance for placing the paper always in a similar position with 

 reference to the sun, must, of course, be superadded. The ob- 

 ject of this contrivance differed from that aimed at, by Sir J. 

 Herschel, in his Aciinometer, which merely measures the solar 

 intensity at the moment of observation j whereas, this is inten- 

 ded as a measure of the aggregate effect of the intensity at the 

 period, be it long or short, during which the paper was submitted 

 to its influence. The interposition of an absorbing fluid has at 

 least this advantage, that it enables the observer to estimate the 

 relative intensity by marking the point at which the paper ceases 

 to be discolored, of which the eye is able to judge more exactly, 

 than of the relative darkness of shade which might be produced 

 on paper exposed unprotected to light of different degrees of bril- 

 liancy. 



Mr. Talbot offered some remarks on Daguerre's photogenic 

 process* M. Arago had stated to the Institute that the sciences 



* See this Jour. 37 ; 374. 

 Vol. xxxYiii, No. 1.— Oct.-Dec. 1839. 13 



