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?

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 60° 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 120° to 130°. 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.">[

Printing Positives.—I will venture to assure Amateur that,—if he will follow Dr. Diamond's formula for albumenizing Canson paper, either positive or negative, viz.,

Chloride of sodium (salt)	5 grs.
Chloride of ammonium	5 grs.
Water	1 oz.
Albumen, or the white of one egg, which is near enough for the purpose	1 oz.

and will excite this paper by floating it for about two minutes on a solution of nitrate of silver twenty grains to the ounce, distilled water,—provided his chemicals are good, he will obtain perfectly satisfactory results.

Let his fixing bath be a saturated solution of hypo. soda, and if newly made let him, as recommended by Dr. Diamond, add 40 grains of chloride of silver to every 8 ounces of the solution. The addition of a grain of sel d'or to every 8 ounces of solution will greatly improve the tones of colour; and if, after some

time, the positives become more of a brown tint than he likes, let him add a small quantity of sel d'or, half a grain to a bath of from 12 to 16 ounces, and he will find the dark tints restored.

I inclose a copy of the print of "Horse-shoeing," obtained precisely by the method described. It is rather overprinted; but if Amateur will give you his address, and you will forward it to him, it will show him what tones of colour and depth may be procured by following the foregoing directions.

C. E. F.

Photographic Excursions.—A few Fellows of the Society of Antiquaries have formed themselves into a Photographic Club for the purpose of making periodical excursions into the country, and so securing accurate views of the objects of antiquarian interest in the different localities they may visit. As it is intended that a copy of every photograph so taken shall be deposited in the portfolios of the Society, the advantages likely to result from this little reunion, both to the Society of Antiquaries and to Archæology generally, are very obvious.