E. F. (Sheffield). _It is only in converting a positive picture into a negative one, or in increasing the powers of a feeble negative, that the bichloride of mercury is recommended to be used. A perfectly good printing negative will be procured by following the instructions we have given in our former Numbers._ DR. DIAMONDS'S Photographic Notes _will treat fully upon this subject_. OUR SIXTH VOLUME, _strongly bound in cloth, with very copious Index is now ready, price 10s. 6d. Arrangements are making for the publication of complete sets of_ "NOTES AND QUERIES," _price Three Guineas for the Six Volumes_. "NOTES AND QUERIES" _is published at noon on Friday, so that the Country Booksellers may receive Copies in that night's parcel, and deliver them to their Subscribers on the Saturday_. * * * * * {195} PHOTOGRAPHY.--Collodion (Iodized with the Ammonio-Iodide of Silver).--J. B. HOCKIN & CO., Chemists, 289. Strand, were the first in England who published the application of this agent (see _Athenæum_, Aug. 14th). Their Collodion (price 9d. per oz.) retains its extraordinary sensitiveness, tenacity, and colour unimpaired for months: it may be exported to any climate, and the Iodizing Compound mixed as required. J.B. HOCKIN & CO. manufacture PURE CHEMICALS and all APPARATUS with the latest Improvements adapted for all the Photographic and Daguerreotype processes. Cameras for Developing in the open Country. GLASS BATHS adapted to any Camera. Lenses from the best Makers. Waxed and Iodized Papers, &c. * * * * * TO PHOTOGRAPHERS.--Pure Chemicals, with every requisite for the practice of Photography, according to the instructions of Hunt, Le Gray, Brébisson, &c. &c., may be obtained of WILLIAM BOLTON, Manufacturer of pure chemicals for Photographic and other purposes. Lists of Prices to be had on application. 146. Holborn Bars. * * * * * PHOTOGRAPHIC PICTURES.--A Selection of the above beautiful Productions may be seen at BLAND & LONG'S, 153. Fleet Street, where may also be procured Apparatus of every Description, and pure Chemicals for the practice of Photography in all its Branches. Calotype, Daguerreotype, and Glass Pictures for the Stereoscope. BLAND & LONG, Opticians, Philosophical and Photographical Instrument Makers, and Operative Chemists, 153. Fleet Street. * * * * * PHOTOGRAPHIC PAPER.--Negative and Positive Papers of Whatman's, Turner's, Sanford's, and Canson Frères' make. Waxed-Paper for Le Gray's Process. Iodized and Sensitive Paper for every kind of Photography. Sold by JOHN SANFORD, Photographic Stationer, Aldine Chambers, 13. Paternoster Row, London. * * * * * PHOTOGRAPHY.--HORNE & CO.'S Iodized Collodion, for obtaining Instantaneous Views, and Portraits in from three to thirty seconds, according to light.