PHOTOGRAPHIC CORRESPONDENCE.
Early Notice of the Camera Obscura.—I send you an early notice of the camera obscura, which is to be found in vol. vi. of the Nouvelles de la République des Lettres for September, 1686, p. 1016. It is taken from a letter of Mons. Laurenti, médecin, of Boulogne, "Sur l'érection des espèces dans une chambre optique."
"C'est ainsi qu'on nomme one chambre exactement fermée partout, si ce n'est dans un endroit par où on laisse entrer la lumière, afin de voir peints, et situés à rebours, sur un morceau de papier blanc, les objets de dehors qui respondent à ce trou, auquel il faut mettre un verre convexe. On a souhaité, pour donner plus d'agrément à ce spectacle, que les objets se peignissent sur ce papier selon leur véritable situation; et pour cet effet on a cherché des expédiens qui redressassent les espèces avant qu'elles parvinssent au foier du verre, c'est-à-dire, sur le papier. L'auteur raporte '10' de ces expédiens, et trouve dans chacun d'eux quelque chose d'incommode, mais enfin il en raporte un autre, qui est exempt de toutes ces incommoditéz, et qui, par le moien d'un prisme, au travers duquel il faut regarder les images peints sur le papier, les montre dans leur situation droite, et augmente même la vivacité de leurs couleurs. C'est le hazard qui a découvert ce phénomène."
This letter is to be found at length in the Miscellanea Curiosa, sive Ephemeridum Medico-Physicarum Germanicarum Academiæ Naturæ curiosorum decuria II. annus quartus, anni 1685 continens celeberrimorum Virorum observationes medicas: Norimbergæ, 1686, in 4to. It may perhaps be worth consulting, if it were only to know what the ten rejected expedients are.
Anon.
Queries on Dr. Diamond's Collodion Process.—Will you oblige me by informing Dr. Diamond through your valuable publication, that I am, in common with many others, extremely indebted to him for his collodion, and would esteem it a favour if he would answer the following Queries, viz.:
1st. He says, in answer to a previous Query, that "nitrate of potassa" is not formed in his process. Now I wish to ask if (as the iodide of silver is redissolved in iodide of potassium) it is not formed when the plate is plunged into the nitrate silver bath, as the nitrate decomposes the iodide of potassium?
2nd. How long will the collodion, according to his formulæ, keep, as collodion made with iodide of silver generally decomposes quickly.
3rdly. Why does he prohibit washed ether?
4thly. Does he think cyanide of potassium would do as well as the iodide, to redissolve the iodide of silver, iodide of potassium being at present so dear?
5thly. In his paper process, does not the soaking in water after iodizing merely take away a portion of iodides of silver and potassium from the paper; or, if not, what end is answered by it?
W. F. E.
Baths for the Collodion Process.—Having lately been assured, by a gentleman of scientific attainments, that the sensitiveness of the prepared collodion plate depends rather upon the strength of the nitrate of silver bath than on the collodion, I am desirous of asking how far the experience of your correspondents confirms this statement. My informant assured me, that if, instead of using a solution of thirty grains of nitrate of silver to the ounce of water for the bath, which is the proportion recommended by Messrs. Archer, Horne, Delamotte, Diamond, &c., a sixty grain solution be substituted, the formation of the image would be the work of the fraction of a second. This seems to me so important as to deserve being brought under the notice of photographers—especially at this busy season—without a moment's delay; and I therefore record the statement at once, as, from circumstances with which I need not encumber your pages, I shall not have an opportunity of trying any experiment upon the point for a week or two.
Upon referring to the authorities on the subject of the best solution for baths, I have been struck with their uniformity. One exception only has presented itself, which is in a valuable paper by Mr. Thomas in the 6th Number of the Journal of the Photographic Society. That gentleman directs the bath to be prepared in the following manner:
Into a 20 oz. stoppered bottle, put—
Nitrate of silver 1 oz.
Distilled water 10 oz.
Dissolve.
Iodide of potassium 5 grs.
Distilled water 1 dr.
Dissolve.
On mixing these two solutions, a precipitate of iodide of silver is formed. Place the bottle containing this mixture in a saucepan of hot water, keep it on the hob for about twelve hours, shake it occasionally, now and then removing the stopper. The bath is now perfectly saturated with iodide of silver; when cold, filter through white filtering paper, and add—
Alcohol 2 drs.
Sulphuric ether 1 dr.
The prepared glass is to remain in the bath about eight or ten minutes. Now, is this bath applicable to all collodion, or only to that prepared by Mr. Thomas; and if the former, what is the rationale of its beneficial action?
A Beginner.