This mode of illumination, being feeble in chemical rays, is best adapted for object-glasses of low power. The exposure required to produce a Negative impression with the one-inch glass may be from three to five minutes. As the sensitive plate would be liable to become dry during that time, it is recommended to coat it with some preservative solution by the modes described in the sixth Chapter. Mr. Crookes having lately shown that the Bromide of Silver is more sensitive than the Iodide to artificial light, a mixture of the two salts may conveniently be used (see pp. 66 and 232). The development may be conducted in the same manner as that for preserved sensitive plates; fixing with Cyanide of Potassium before the development is fully complete, if any tendency to fogging is observed (see page 224). The Rev. W. Towler Kingsley has communicated a process by which very beautiful Microscopic Photographs have been obtained. He illuminates (in the absence of sunlight) with the brilliant light produced by throwing a jet of mixed Oxygen and Hydrogen gases upon a small cone of Lime or Magnesia. Particular stress is laid upon the object-glass of the Microscope being a good one for the purpose; and indeed all who have given attention to the subject are agreed upon this point--that there is a considerable difference in the Photographic value of objectives, and this independent of the angular aperture of the glass. CHAPTER IV. THE PRACTICAL DETAILS OF PHOTOGRAPHIC PRINTING. This Chapter is divided as follows:-- Section I.--The ordinary direct process of positive printing. Section II.--Positive printing by development. Section III.--The mode of toning Positives by Sel d'or. Section IV.--On printing enlarged or reduced Positives, transparencies, etc. SECTION I. _Positive Printing by the direct action of Light._ This includes--the preparation of sensitive paper,--of fixing and toning Baths,--and the manipulatory details of the process. _Selection of Paper for Photographic Printing._--The ordinary varieties of paper sold in commerce are not well adapted for the production of Positive prints. Papers are manufactured purposely which are more smooth and uniform in texture. Many samples of even the finest paper are however defective, and hence each sheet should be examined separately by holding it against the light, and if spots or irregularities of texture are seen, it should be rejected. These spots usually consist of small particles of brass or iron, which, when the paper is rendered sensitive, decompose the Nitrate of Silver and leave a circular mark very noticeable after fixing.