PHOTOGRAPHIC CORRESPONDENCE.
A Stereoscopic Note.—I possess a small volume entitled A Disquisition about the Final Causes of Natural Things, by T. H., B.B., Fellow of the Royal Society, 1688. "To which are subjoined, by way of Appendix, some uncommon observations about vitiated sight." In this strange appendix, one of the uncommon observations is worth the notice of your correspondents who write on stereoscopic subjects. I give you an extract from it:
"It has been of late the opinion of very learned men, that though both our eyes are open, and turned towards an object, yet 'tis but one of them at a time that is effectually employed in giving us the representation of it: which opinion, in this place where I am writing but observations, it were not proper to discuss, especially because what is suppos'd to be observed will not always uniformly happen, but may vary in particular persons according to their several customs, and the constitution of their eyes: for I have, by an experiment purposely made, several times found, that my two eyes together see an object in another situation than either of them apart would do." And in giving instances for and against binocular vision, the author says: "A yet more considerable instance of such mistakes I afterwards had from a noble person, who, having in a fight, where he play'd the hero, had one of his eyes strangely shot out by a musquet bullet, that came out at his mouth, answered me, that not only he could not well pour drink out of one vessel into another, but had broken many glasses by letting them fall out of his hand, when he thought he had put them into another's, or set them down upon a table." The whole book is a very curious one, and I should be obliged if the Editor of "N. & Q." could tell me who T. H. was?[[5]]
J. Lawson Sisson.
Edingthorpe.
Footnote 5:[(return)]
The Hon. Robert Boyle.
Photographic Query.—I think many amateur photographers would be thankful for plain and simple directions how to mount their positives on cardboard. Would the Editor of "N. & Q." assist us in this?
J. L. S.
Deepening Collodion Negatives.—I have lately been trying a method of deepening collodion negatives, so as to render instantaneous impressions capable of being printed from, which I have found to answer admirably;
and although it is but a slight modification of Mr. Lyte's process described in "N. & Q.," it is a very important one, and will be found to produce far better results. The picture having been developed in the usual way, with a solution of pyrogallic acid, is whitened by means of Mr. Archer's solution of bichloride of mercury. The plate is then washed with water and a solution of iodide of cadmium poured on. This converts the white chloride of mercury, which constitutes the picture, into the yellow iodide, in the same manner as the solution of iodide of potassium recommended by Mr. Lyte; but is much to be preferred, as it produces a more uniform deposit. The solution of iodide of potassium dissolves the iodide of mercury as soon as it is formed, and therefore cannot be left on the plate until the decomposition of the chloride is complete, without injury resulting to the picture, as the half-tones are thereby lost, and those parts over which the solution first flows become bleached before the other parts have attained their highest tone; whereas the solution of iodide of cadmium may be allowed to remain for any length of time on the plate, without any fear of its injuring the negative.
J. Leachman.
Caution to Photographers.—About six months since, I procured some gun cotton from a chemist which appeared very good, being quite soluble, and the collodion produced by it was excellent. That which I did not use I placed in what I believed to be a clean dry-stopped bottle, and put the bottle in a dark cupboard. I was much surprised the other day, upon going to the cupboard, to find the stopper blown out, and the cotton giving out dense red fumes of nitrous acid. It appears to me to be almost upon the point of combustion, and I have, accordingly, placed it under a bell-glass in a porcelain dish to watch the result. I feel satisfied, however, that there is some risk, and, as it may often be near ether, spirits of wine, or other inflammable chemicals, that caution is necessary not only in preserving it at home, but especially in its transmission abroad, which is now done to some extent.
An Amateur.