When it is intended that the pictures taken are to be viewed by an instrument that requires their distance from the eyes to be _less_ than the focal length of the lens used in their formation, what is the result? Why, that they subtend an angle larger than in nature, and are consequently apparently _increased_ in bulk; and the obvious remedy is to _increase_ the angle between the points of generation in the exact ratio as that by which the visual distance is to be lessened. There is one other consideration to which I would advert, viz. that as we judge of _distance_, &c. mainly by the degree of _convergence_ of the optic axes of our two eyes, it cannot be so good to arrange the camera with its two positions quite parallel, especially for objects at a short or medium distance, as to let its centre radiate from the principal object to be delineated; and to accomplish this desideratum in the readiest way (for portraits especially), the ingenious contrivance of Mr. Latimer Clark, described in the _Journal_ of the Photographic Society, appears to me the best adapted. It consists of a modification of the old parallel ruler arrangement on which the camera is placed; but one of the sides has an adjustment, so that within certain limits any degree of convergence is attainable. Now in the case of the pictures alluded to by MR. H. WILKINSON in Vol. viii., p. 181., it is probable they were taken by a camera placed in two positions parallel to one another, and it is quite clear that only a _portion_ of the two pictures could have been really stereoscopic. It is perfectly true that two indifferent negatives will often combine and form one good stereoscopic positive, but this is in consequence of one possessing that in which the other is deficient; and at any rate two _good_ pictures will have a _better_ effect; consequently, it is better that the two views should contain exactly the same _range_ of vision. GEO. SHADBOLT. _Protonitrate of Iron_.--"Being in the habit of using protonitrate of iron for developing collodion pictures, the following method of preparing that solution suggested itself to me, which appears to possess great advantages:-- Water 1 oz. Protosulphate of iron 14 grs. Nitrate of potash 10 grs. Acetic acid 1/2 drm. Nitric acid 2 drops. In this mixture nitrate of potash is employed to convert the sulphate of iron into nitrate in place of nitrate of baryta in Dr. Diamond's formula, or nitrate of lead as recommended by Mr. Sisson; the advantage being that no filtering is required, as the sulphate of potash (produced by the double decomposition) is soluble in water, and does not interfere with the developing qualities of the solution.