It must be premised however, that the same degree of excellence is not to be expected in a Negative Photograph which has been improperly developed in the first instance and more especially if the exposure to light was too short. Any "instantaneous Positive" may be rendered sufficiently intense for a Negative, but in that case the shadows are almost invariably imperfect. 1. _Treatment of the image with Sulphuretted Hydrogen or Hydrosulphate of Ammonia._--The object is to convert the metallic Silver into _Sulphuret of Silver_, and if this could be done it would be of service. The mere application of an Alkaline Sulphuret has however but little effect upon the image, excepting to darken its surface and destroy the Positive appearance by reflected light; the structure of the metallic deposit being too dense to admit of the Sulphur reaching its interior. Professor Donny ('Photographic Journal,' vol. i.) proposes to obviate this by first converting the image into the white Salt of Mercury and Silver by the application of Bichloride of Mercury, and afterwards treating it with solution of Sulphuretted Hydrogen or Hydrosulphate of Ammonia. Negatives produced in this way are of a brown-yellow colour by transmitted light, and opaque to chemical rays to an extent which would not, _à priori_, have been anticipated. 2. _MM. Barreswil and Davanne's process._--The image is converted into Iodide of Silver by treating it with a saturated solution of Iodine in water. It is then washed--to remove the excess of Iodine,--exposed to the light, and a portion of the ordinary developing solution, mixed with Nitrate of Silver, poured over it. The changes which ensue are precisely the same as those already described; the whole object of the process being to bring the metallic surface back again into the condition of Iodide of Silver modified by light, that the developing action may be commenced afresh, and more Silver deposited from the Nitrate in the usual way. 3. _The process with Bichloride of Mercury and Ammonia._--The image is first converted into the usual white double Salt of Mercury and Silver by the application of a solution of the Corrosive Sublimate. It is then treated with Ammonia, the effect of which is to _blacken_ it intensely. Probably the alkali acts by converting Chloride of Mercury into the black Oxide of Mercury. In place of Ammonia, a dilute solution of Hyposulphite of Soda or Cyanide of Potassium may be used, with very similar results. CHAPTER VIII. ON THE THEORY OF POSITIVE PRINTING. The subject of Collodion Negatives having been explained in the previous Chapter, we proceed to show how they may be made to yield an indefinite number of copies with the lights and shadows correct as in nature. Such copies are termed "Positives," or sometimes "Positive prints," to distinguish them from direct Positives upon Collodion.