In a tolerably bright day in the spring or summer months, and with a newly-mixed Collodion, allow four seconds for a Positive Portrait, and eight seconds for a Negative. With a double-combination Lens of large aperture and short focus, perhaps three seconds, and six seconds, or even less, may be sufficient. In the dull winter months, in the smoky atmosphere of large cities, or when using an old Collodion brown from free Iodine, multiply these numbers three or four times, which will be an approximation to the exposure required. It is by the appearance presented under the influence of the developer, which will immediately be described, that the operator ascertains the proper time for exposure to light. THE DEVELOPMENT OF THE IMAGE. The details of developing the latent image differ so much in the case of Positive and Negative pictures, that it is better to describe the two separately. The development of direct Positives.--With Sulphate of Iron as a developer, it is most simple to develope the image by immersion. The solution may conveniently be poured into a vertical trough, such as that used for exciting, and the plate immersed by means of a glass dipper in the usual way. Unless the weather be cold, the image makes its appearance in three or four seconds, and the film is then immediately washed with clear water. Whilst in the Bath, the plate is kept in gentle motion, and the operator must not expect to see the image very distinctly, except the high lights; the shadows, being faint, are partially concealed by the unaltered Iodide, but they come out during the fixing. The action of the Sulphate of Iron is stopped at an early period, or an excess of development will be incurred. The Bath may be used repeatedly. In using Pyrogallic Acid or Nitrate of Iron to develope glass Positives, the plate may be placed upon a levelling-stand, or held in the hand, or by the pneumatic holder, and the solution poured on quickly at one corner; by blowing gently or inclining the hand, as the case may be, it is scattered evenly over the film before the development commences. If any difficulty is experienced in covering a plate evenly with a strong developer before the action commences, it may be overcome by using a shallow cell formed by cementing two or three thicknesses of window-glass on a piece of patent plate to the depth of a quarter of an inch. The size of the cell should be only slightly larger than the plate intended to be developed, that the waste of fluid may be as little as possible. The cell is held in the left hand, and the plate being placed in it, a sufficient quantity of the developer is poured on at one corner. By a slight inclination, the fluid is caused to flow in a uniform sheet over the surface of the film, backwards and forwards. The image starts out quickly, and the developer is then at once poured off, and the film washed as before.