MR. R. W. THOMAS begs most earnestly to caution photographers against purchasing impure chemicals, which are now too frequently sold at very low prices. It is to this cause nearly always that their labours are unattended with success. Chemicals of absolute purity, especially prepared for this art, may be obtained from R. W. THOMAS, Chemist and Professor of Photography, 10. Pall Mall. N.B.--The name of Mr. T.'s preparation, Xylo-Iodide of Silver, is made use of by unprincipled persons. To prevent imposition each bottle is stamped with a red label bearing the maker's signature. * * * * * TO PHOTOGRAPHERS.--MR. PHILIP DELAMOTTE begs to announce that he has now made arrangements for printing Calotypes in large or small quantities, either from Paper or Glass Negatives. Gentlemen who are desirous of having good impressions of their works, may see specimens of Mr. Delamotte's Printing at his own residence, 38. Chepstow Place, Bayswater, or at MR. GEORGE BELL'S, 186. Fleet Street. * * * * * 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. * * * * * PHOTOGRAPHY.--HORNE & CO.'S Iodized Collodion, for obtaining Instantaneous Views, and Portraits in from three to thirty seconds, according to light. Portraits obtained by the above, for delicacy of detail rival the choicest Daguerreotypes, specimens of which may be seen at their Establishment. Also every description of Apparatus, Chemicals, &c. &c. used in this beautiful Art.--123. and 121. Newgate Street. * * * * * ROSS'S PHOTOGRAPHIC PORTRAIT AND LANDSCAPE LENSES.--These lenses give correct definition at the centre and margin of the picture, and have their visual and chemical acting foci coincident. _Great Exhibition Jurors' Reports_, p. 274. "Mr. Ross prepares lenses for Portraiture having the greatest intensity yet produced, by procuring the coincidence of the chemical actinic and visual rays. The spherical aberration is also very carefully corrected, both in the central and oblique pencils." "Mr. Ross has exhibited the best Camera in the Exhibition. It is furnished with a double achromatic object-lens, about three inches aperture. There is no stop, the field is flat, and the image very perfect up to the edge." Catalogues sent upon Application. A. ROSS, 2. Featherstone Buildings, High Holborn. * * * * * Just published, price 1s., free by Post 1s. 4d.,