various bodies to the Chemical Rays. 139 



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 J 839. 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 town, who, having di- 

 rected much of his attention to optics, furnished me with in- 

 formation and instruments by which my progress was greatly 

 accelerated'i^. 



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 Becquerel's experiments in the 

 Athenaeum, No. 621, I am perfectly unacquainted v/ith 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^ Electricitii 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. 



