328 The Kcv. J. B. Keade un some earhj 



I emploj'cd hyposulphite of soda as a fixer. Mr. Hodgson, an 

 able practical chemist at Apothecaries' Hall, assisted me in the 

 preparation of this salt, which at that time was probably not to 

 be found; as an article of sale, in any chemist's shop in London. 

 Sir John Herschel had previously announced the peculiar action 

 of this preparation of soda on salts of silvei', but I believe that I 

 was the first to use it in the processes of photography. I also 

 used iodide of potassium, as appears from my letter, as a fixer, 

 and I employed it as well to form iodide of lead on glazed cards 

 as an accelerator. Iodide of lead has of itself, as I form it, con- 

 siderable photographic properties, and receives very fair impres- 

 sions of plants, lace, and drawings when placed upon it, but with 

 the addition of nitrate of silver and the infusion of galls the 

 operation is perfect and instantaneous. Pictures thus taken 

 were exhibited at the Royal Society before Mr. Talbot proposed 

 his iodized paper. The microscopic photographs exhibited at 

 Lord Northampton's in 1839 i*emained in his lordship's posses- 

 sion. I subsequently made drawings of sections of teeth ; and 

 one of them, a longitudinal section of a tooth of the Lamna, was 

 copied on zinc by Mr. Lens Aldous for Owen's ' Odontography.' 

 I may say this much as to my own approximation to an art, 

 which has deservedly and by universal consent obtained the name 

 of Talbotype. 



Sir David Brewster, in his ' History of Photography,' passes 

 immediately from the experiments of Wedgwood to those of 

 Talbot ; but the Transactions of the Royal Society, to which my 

 friend Mr. Gravatt has directed my attention, will enable us to 

 insert, if not a chapter, at least a very pregnant parenthesis. 

 The Bakerian Lecture, in 1803, by Dr. Young, who never 

 touched a subject without leaving his mark upon it, contains a 

 highly interesting and original experiment on the photographic 

 representation of the invisible chemical rays beyond the blue end 

 of the spectrum. This experiment does not happen to be recorded 

 in the first edition of your ' Researches on Light ; ' but no one 

 will refer to it with greater pleasure than yourself, not only 

 because it is the first photographic analysis of the spectrum, but 

 also because it has the higher merit, even as it stands alone, of 

 being the one sufficient fact which establishes the consummation 

 so devoutly looked for, at the conclusion of your work, from the 

 perservering accumulation of facts only; for it is in itself a 

 sim])le and demonstrative proof, to use the words of Dr. Young, 

 of the general law of interference, and, in your own words, 

 "reconciles the chemical action of the photographic force, energia, 

 with the undulatory theory of liyht." Dr. Young's experiment 

 forms the conclusion of his lecture, and is given in the following 

 terms : — "The existence of solar rays accompanying light more 



