"I employed 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 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 silver, 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, considerable photographic properties, and receives very fair impressions 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 remained in his lordship's possession. 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." _Photographic Cautions._--Diffused light being one of the most common causes of photographic failures, I beg to call the attention of your readers to the construction of their cameras. Working with a friend, and taking the same localities, using the same paper and chemicals, his pictures have proved comparative failures, a general browning pervading the whole, evidently the effect of light. Every inspection failed to discover it, until the mode was adopted of putting one of the paper-holders in its position as for taking a picture, then removing the lens, and, with the aid of the focussing-bag, looking through the hole where the lens is applied, when light became visible in many spaces, entirely accounting for these failures. As many such cameras are now becoming made upon the same sliding construction, every one should test his apparatus before he commences, for such a one is entirely useless. Lately also the glass corners for collodion plate-holders in the dark slides, have been by some makers replaced by a sort of silver _looking_ wire, but possessing little of that metal. The most minute portion of the copper in this wire coming in contact with the excited collodion, produces a decomposition sufficient to spoil any picture. These may appear trivial things to "make a note of," but as they have caused much vexation to one who has had some photographic experience, they may still more perplex a novice; and as you have done so much towards making the science plain, I hope you will give them space in your forthcoming Number. LUX IN CAMERA.