_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.
_A Query respecting Collodion._--I have been making some collodion by Mr. Tery's process, and have iodized it with a very sensitive medium. The collodion is very clear and properly diluted. The ether I used had a very powerful smell of sulphur, and was likewise very strong and volatile. I diluted it with an equal { 526} volume of alcohol. The ether was then still very strong. The cotton dissolved freely. On mixing the iodizing medium, the colour of the collodion turns immediately to nearly a port-wine colour, but still remains very clear. I obtain a very good film of iodide of silver from the bath, but cannot produce a picture under five or seven minutes, whereas with the same lens, and the same iodizing medium, viz.