On comparing various authors, and _reducing_ their directions to a standard of _one ounce of ether_, I find the following differences: viz., DR. DIAMOND (Vol. vi., p. 277.) prescribes _about_ three grains of gun cotton; Mr. Hennah (_Directions, &c._, p. 5.) about seven grains; the Count de Montizon (_Journ. of Phot. Soc._, p. 23.) eight grains; whilst Mr. Bingham (_Supplement to Phot. Manip._, p. 2.) directs about _thirty-four_ grains! in each case to a single ounce of ether. These differences are too wide to come within even Mr. Archer's "long range," that "the proportions ... must depend entirely upon the strength and the thickness required ... the skill of the operator and the season of the year." (Archer's _Manual_, p. 17.) COKELY. _Filtering Collodion._--Count de Montizon, in his valuable paper on the collodion process, published in the second number of the _Journal of the Photographic Society_, objects to filtration on the ground that the silver solution is often injured by impurities contained in the paper. It may be worth while to state, that lime, and other impurities, may be removed by soaking the filter for a day or two, before it is used, in water acidulated with nitric acid; after which it should be washed with hot water and dried. T. D. EATON. _Photographic Notes_ (Vol. vii., p. 363.).--I wish to correct an error in my communication in "N. & Q." of April 9: in speaking of "a more _even_ film," I meant a film more _evenly sensitive_. I am sorry I have misled MR. SHADBOLT as to my meaning. I have very rarely any "spottings" in my pictures; but I always drop the plates once or twice into the bath, after the two minutes' immersion, to wash off any loose particles. I also drain off all I can of the nitrate of silver solution before placing the glass in the camera, and for three reasons:--1. Because it saves material; 2. Because the lower part of dark frame is kept free from liquid; 3. Because a "flowing sheet" of liquid must interfere somewhat with the passage of light to the film, and consequently with the sharpness of the picture. I think it is clear, from MR. SHADBOLT'S directions to MR. MERITT, that it is no very easy thing to cement a glass bath with marine glue. J. L. SISSON. _Colouring Collodion Pictures_ (Vol. vii., p. 388.).--In your impression of April 16, there is a typographical error of some importance relative to lifting the collodion in and out of the bath: "The plate, after being plunged in, should be allowed to repose quietly from twenty to thirty _minutes_," &c. This should be _seconds_. The error arose, in all {415} probability, from my having used the contractions 20" to 30".