* 
"= 
British Association for the Advancement of Science ‘oT fe. 
hoped to obtain a nunmeridal Sheets of the intensity of we & 
light at different periods of day, and in different parts of the globe. 
It consisted of a sheet of photogenic paper, moderately sensible, 
tolled 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- 
hess, 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 tolight, 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 
teference 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 Actinometer, which merely measures the solar 
intensity at the moment of observation; whereas, this is inten-° 
ed 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 
Say exposed unprotected to light of dierent degre of bril- 
Mr. ey offered some remarks on Daguerre’: s ‘photog thie 
Process.* M. Arago had stated to the Institute that the sciences 
ea * 
* See this Jour. 37; 374. 
Vol. xxxvit, No. 1.—Oct.-Dec. 1839. 1 
