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^-sh utters, 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, T 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 the 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. 
