139 
various bodies to the Chemical Rays. 
that I read Mr. Talbot’s paper on photographic drawing, 
which opened to me new views, and pointed out paths rich 
in the promise of important results. 
The vast sum of delight which the pursuit of this subject 
during the past year has afforded me, makes me a large debtor 
to that erudite gentleman, which I thus humbly, but sincerely, 
acknowledge. 
My first endeavours in the photographic art were directed 
to restoring the natural order of light and shadow ; and 1 
fortunately succeeded in effecting this very early in the sum- 
mer of 1839. My next were to improve the camera for pho- 
tographic purposes, in which object I was most materially 
assisted by Mr. John Towson, of this tov^^n, who, having di- 
rected much of his attention to optics, furnished me with in- 
formation and instruments by which my progress was greatly 
accelerated*. 
Having, in conjunction with this gentleman, while trying 
a variety of lenses, been often perplexed by the dissimilar 
results obtained on the same paper from different kinds of 
glass, I was induced to commence a series of experiments on 
the interference of transparent bodies to the permeation of 
chemical light. 
The same subject has, I am informed, engaged the atten- 
tion of two scientific inquirers on the continent ; but beyond 
a brief notice of M. Edmond BecquerePs experiments in the 
Athenaeum, No. 621, I am perfectly unacquainted with the 
methods or results of their observations. 
Being anxious to obtain a measure of the interference of 
the various bodies I was about to examine, I constructed a 
very delicate galvanometer — the coil being of ribbon copper 
and the needles of French watch-spring. To this instrument I 
connected, by'platina wires, a U tube, as suggested by M.Bec- 
querel in hisTraite d! Rlectricite^ which held in one arm a solu- 
tion of nitrate of silver, and in the other a solution of iodide of 
potassium. Every part of the tube was screened from light, 
except the lowest point, at which the fluids met. On this 
point, by means of a powerful lens, a concentrated pencil of 
light was thrown, which was made to pass through the bodies 
to be examined. The force of electro-chemical action being 
dependent on the quantity of chemical light impinging on 
the exposed portion of the fluids, led me naturally to con- 
clude that the deflections of the needle would furnish very 
accurate comparative results. I have also tried the plan M. 
E. Becquerel adopts, of floating one photometric fluid upon 
* See L. and E. Phil. Mag. for November last, vol. xv. p. 381 . — Edit. 
