Leave in this for half an hour, and then keep it in running water for several hours. If the water is hot, the time of soaking may be lessened: boiling water is objectionable. Nearly dry the positive between sheets of clean blotting-paper, and finish it by passing a very hot iron over it. _General Remarks._--The albumenised paper will keep any length of time in a dry place. When made sensitive, as directed, it will keep three days, always supposing that it is both prepared and kept most carefully excluded from white light. If, instead of a solution of nitrate of silver of ninety grains to the ounce, a weaker one be used, to make the paper sensitive, it will keep when sensitive a much longer time,--with a thirty-grain solution, a fortnight, or sometimes even a month; but then it does not give a positive of the same force and tone as that obtained with the stronger solution. After the fixing bath has done its day's work, it should be poured back into the bottle from which it came, and the bottle be filled up from the finishing bath; and so the bath is kept always of the same quantity; and by adding from time to time chloride of gold, it is kept of the same quality. The nitrate of silver and chloride of silver will never have to be renewed. The iodide of silver should be added as at first, viz. ten drops for about every two hundred positives fixed; and the acetic acid, viz. two drops for about every four hundred. In a bath of twenty-four ounces, as many as thirty positives, five inches by four, may be placed at one time: but the dark tints will then appear very slowly and gradually. To insure a good positive, next to having a good negative, it is most important to print of the right depth, neither too much nor too little. Great attention should be paid to this: for the finest tints are only to be obtained in positives exposed exactly the right time. Positives printed in a bright sun quickly are always better than those obtained by longer exposure without sun. H. P. 21. Maddox Street, Regent Street. _Test for Lenses._--In applying the methods recommended in your last Number for the purpose of testing lenses, there is one precaution absolutely necessary to be taken, but which all your correspondents have omitted to point out. The operator must take care that his _focussing-glass_ is placed at precisely the same distance from the lens as the _collodionised_ glass is. To insure this, my practice is to place a piece of ground glass in the dark frame, which is afterwards to receive the collodionised glass, and to obtain the focus of the lens on that; then to put in the proposed plate, and obtain an impression as described by MR. SHADBOLT. In this way I secure myself from what I believe is often a source of fallacy in these experiments, and am sure that I give the lens a fair trial. E. S.