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11 duration of the reduction effected will vary wirh the energy of the organic reactions, eventually yielding to the latter, and to the re-establishment of the density, and its increase. As I suppose such a collodion—because of its not keeping well in its plain state, but sensitive while it did keep—to be made up in small quantities, and salted with ammonium and cadmium, the most sensitive combinations I know of, which stands next to cadmium collodion ; which admits of the free use of bromide, and which can be modified in sensitiveness, and in its effect upon the said collodion by the increase of one salt or the other, I am led to infer that some such case has made successful operators believe that the presence of the bromide in large quantities has played the important part of sensitiveness, when indeed it was rather attributable to the combination, including an alkaline salt, to the excitation caused, and to the collodion having been employed while that excitation lasted. If, however, the bromide was the cause of it, any bromide should answer; and if the bromide was ammonium, it should be replaced by bromide cadmium, and a trial given". If the collodion were fit to be salted with iodide and bromide cadmium conjointly, the sensitiveness would be deferred appreciably, although more durable when reached. But if not of it, insensitiveness and destruction of the collodion would ensue, I do not say it would be impossible to make a collodion fit for salting with equal parts of iodide and bromide of cadmium; this would, however, be making a collodion to salt in a particular manner, and it cannot there fore effect the result of my experiments, establishing that bromide causes insensitiveness in proportion to its presence, for the different iodised and bromo-iodised collodions, from one sample, were used by me throughout their various states of sensitiveness, past their age of ripeness, and into their period of deterioration, and the best results of each re corded. Nevertheless, I say that wherever bromide can be introduced it is an advantage, as regards the reduction of excessive density, and in its capacity for faithfully rendering foliage, &c. But that it is impossible to work rapidly without the use of large quantities of bromide, is, in my opinion, an error, the dissemination of which will cause, and has caused, many disappointments. The system of parading success, and attributing it to particular causes which arc not the whole truth, is, to say the least of it, disingenuous. If the rapid operators, instead of attributing their success to the use of large quantities of bromide, were to explain how their collodion was made, the treatment it should receive, and its properties of keeping &c., and give a detailed method of working, it would then be the whole truth, and nothing but the truth. Until this is done, which no one need expect, it behoves us to accept their statements and proofs with much caution, especially when the results of careful experiments and extended experience are opposed to them. For in your own columns you have recommended infinitessimal doses of bromide, improperly ridiculed by Mr. Sutton, while many eminent photographers reject the salt altogether. To the mass of beginners, it is as well to point out that the use of large quantities of bromide, on which rapid operators claim their success, has, doubtless, some favour able conditions in the collodion for its presence, and in such case is a necessity of the particular method of working adopted—not the resuscitation, or successful applica tion of a long neglected principle, applicable under every ordinary circumstance. In other words, the practice is not a radical photographical, but local necessity; and that, when ever followed without a perfect knowledge of the working of the rapid operator, disappointment and failure are always possible. 1 he next mail will arrive a fortnight hence, and I hope to read the commencement of the discussion on this bromide and iodide question, and eventually the decision which will be arrived at by wiser heads than mine.—I am, yours truly, Augustus Webb. Aqotographit Zlotes and QQueries. To Secure Clouds in Landscapes. Sir,—As the desire for securing natural clouds and proper atmospheric effects in landscapes is now happily increasing, I beg to acquaint you with a small instrument which I have lately been endeavouring to construct for facilitating this object, and would feel greatly obliged by your opinion as to its prob able utility, through the medium of your answers to correspen- dents, as you will be, no doubt, better able to judge than I am. In the case of instantaneous views, or those taken over water, where the horizon is a straight line, I suppose an excess of exposure may he given to the foreground quite easily, by the ordinary Hap shutter; but in the majority of landscapes, where a much longer period of exposure is requisite, and the horizon is seldom a straight line, the light cannot well be cut oft" from the sky by these means, without the more elevated portions of the foreground suffering from want of exposure. The small instrument I have constructed, with the view of remedying this evil, consists of a folding hood placed on to the end of the lens-tubes, in front of the camera, and carrying in front a frame filled with a number of small slips, which fit into each other in such a manner as to form a light-proof covering, and admit of sliding up and down with a small amount of friction. To use this the operator has simply to put his lenses in focus, and while he is looking at the image on the ground glass, to run his finger along the tops of these slips, and gently press them down till they form such an outline as to exactly cut off all the light from the sky; and then, when the foreground has had sufficient exposure, the frame is removed out of the way, and an instantaneous exposure given over all the plate, just sufficient for the sky and clouds. The outline these pieces make is slightly serrated, but that assists in softening off the light more gradually. The light may be still more gradually cut off, if need bo, by gently lifting up the frame, Or turning it round some time, during the exposure of the foreground. As the latter part of the afternoon is the only time when I can attend to photo graphy, and as this neighbourhood is always enveloped in mist and smoke at this season, I have not had an opportunity of giving this contrivance a fair trial; but such trials as it has got lead mo to think that only conditions favourable for photo graphy are required for success. The enclosed, which I certainly cannot call pictures, were taken in the evening, and are sadly blemished by mist, smoke, and wind. The foreground was exposed about a hundred times as long as the clouds. The contrivance could be combined with the instantaneous shutter, so as to form one instrument. It seems to me to make a considerable difference to the fore ground, especially with a prolonged exposure and compound lens, as it cuts off in such cases all the light from the sky during the exposure of the foreground. This difference is most striking when the lenses are pointed towards the sun. Even in that case all sorts of haloes seem to be prevented. Perhaps, in cases whore there are no clouds to take, it might be found use ful for improving the foreground of the picture, and producing a sky of any desired density, to admit of painting in clouds—or in the case of dry plates.—I am, sir, your obediont servant, Bruce Castle, 29th December, 1862. Alpha. [If we remember rightly, the late Mr. Scott Archer was in the habit of using a method of securing a short exposure to the sky by a method similar in principle but different in detail. A piece of paper was torn or cut, with an edge following the outline of the horizon. This was fixed in a frame inside the camera, in such a way as to screen the sky during the greater part of the exposure, and turned down or removed for a few seconds at the close, so as to give a brief exposure to the sky. The object is so good that we hail every effort to facilitate it with pleasure. If there be one purpose we have more steadily kept before us than another, during the last few years, it has been to aid in the abolition of the rmmeaning patch of white paper which has so long done duty as a sky in photographic landscapes, to the destruction alike of atmosphere, breadth, or real brillancy. The two examples enclosed very satisfactorily illustrate that the contrivance of our correspondent will ad mirably facilitate the production of natural clouds in conjunc tion with a foreground requiring long exposure.— ED.]