140 Mr. Hunt on the Permeability of 



another. Experience has, however, convinced me that the 

 galvanometer, although capable of being made in the hands 

 of a skilful manipulator a very accurate measurer of the diur- 

 nal variations of the quantity of chemical rays in the solar 

 beam*, cannot be depended on where a series of nice com- 

 parisons are required. I have never yet been enabled to ar- 

 rive at precisely the same results by this instrument in any 

 two sets of observations; every thin cloud or the lightest 

 smoke materially altering the deflections. I have, however, 

 found it of use in giving me near approximations to a correct 

 arrangement. I proceed thus : having by the galvanometer 

 tabulated a number of bodies, I select those whose interfe- 

 rence seems to approach near each other, and place them in 

 regular order, under the same circumstances, upon a sheet of 

 highly sensitive photographic paper in a dark room ; then 

 opening the window-shutters, expose it for three minutes to 

 the direct influence of the solar rays, or for twice that time 

 to diffused daylight ; again darkening the apartment I ex- 

 amine the tints at which the paper has arrived under each 

 body, and mark their correspondence or otherwise with the 

 observations by the galvanometer. By carefully repeating 

 many times each set of experiments, I am enabled to correct 

 small errors of observation. 



I use yet another method to test the correctness of the 

 foregoing processes, which consists in filling a camera with 

 the fluid or gas to be examined, or interposing the solid body 

 and receiving the sun's image on a disc of silvered copper, 

 prepared according to the principles of the Daguerreotype. 



As many simple contrivances will suggest themselves to 

 those who are desirous of repeating the experiments, it may be 

 sufficient for me to state, that my apparatus is simply one 

 cylinder sliding within another for the purpose of adjusting 

 the focus to the different densities of the bodies, and that the 

 photographic disc is protected from the fluid or gas by a 

 piece of tested plate glass well greased around the edges, as 

 are also the cylinders, throughout their length. 



This plan may appear open to some of the objections I have 

 urged against the galvanometer ; but, as from the sensitiveness 

 of the preparation an exposure of thirty seconds is sufficient, 

 you are enabled to select your moments of observation, and 



* Long prior to th-e publication of the speech of M. Arago on the re- 

 port of the Commission on the Daguerreotype, both Mr, Towson and 

 myself had remarked that the light of morning acted more powerfully on 

 photographic preparations than the evening light. The paper which at 

 nine in the morning became in ten minutes a rich purple bronze, took 

 aim ost twice that time to reach the same hue at three in the afternoon. 



