* * * * * PHOTOGRAPHIC CORRESPONDENCE. _Photographic Query._--Given the diameter and focal length of a simple achromatic lens; at what distance from it must a diaphragm of given diameter be placed to give the best possible image? O. _Improvement in Collodion._--As there are many photographers who are not members of the Photographic Society, and who do not see the journal published by that body, a statement of what I think will be found a very material improvement in the manufacture of collodion may not be unacceptable to the readers of "N. & Q." To five drachms of pure _washed_ ether, add one drachm alcohol 60deg over proof, and dissolve therein sufficient soluble cotton to make it of the consistence of oil (the exact quantity must depend rather upon the dexterity of the operator, as the thicker it is the more difficult to use) then add twenty minims of chloroform, dropping in the latter, which will fall to the bottom, but is readily dissolved on shaking the mixture for a few minutes. To two drachms of the same alcohol add the iodizing material preferred, and mix with the other ingredients. The above will be found to flow very evenly smoothly over the plate; is tough, intense, and _structureless_ in appearance. I have not yet determined what is the best iodizing mixture, but at present I prefer iodide of potassium _alone_, if pure, and twenty grains to the ounce of alcohol is the proportion I generally adopt; thus having five grains in each ounce of collodion. Lastly, as regards the soluble cotton, I cannot find any better material than that produced according to the formula published by Mr. Hadow, in the March Number of the _Photographic Journal_, thus: "Take of nit. potash, five parts; sulphuric acid, ten parts; water, one part; _all by weight_. Add the water to the nitrate of potash, and then the acid, and immediately immerse as much cotton wool as can be thoroughly saturated by the mixture, leaving it in for _at least_ ten minutes, and wash with a great abundance of water. The object of adding the cotton immediately that the acid has been mixed with the nitrate of potash, is to expose it to the action of the chemicals while they are at a temperature of from 120deg to 130deg. For farther particulars on this head, I must refer to Mr. Hadow's paper. GEO. SHADBOLT. [This application is not a novelty to us: DR. DIAMOND has for some time added a small portion of his amber varnish (which is prepared from chloroform) to his collodion, and with satisfactory results. It is a pity that so admirable a varnish is not to be procured at the generality of photographic warehouses. We have never yet been able to procure any which will bear comparison with some which DR. DIAMOND was good enough to prepare for us.--ED. "N. & Q."]