After the plate was prepared and dried, it was exposed in the camera, or by superposition, under a print, or other suitable subject, that would lie flat. For the latter, an exposure of two or three hours in bright sunshine was necessary, and the former required six or eight hours in a strong light. Even those prolonged exposures did not produce a visible image, and the resultant picture was not revealed to view until after a tedious process of dissolving, for it could scarcely be called development. M. Niépce himself says, "The next operation then is to disengage the _shrouded_ imagery, and this is accomplished by a solvent." The solvent consisted of one measure of the essential oil of lavender and ten of oil of white petroleum or benzole. On removing the tablet from the camera or other object, it was plunged into a bath of the above solvent, and left there until the parts not hardened by light were dissolved. When the picture was fully revealed, it was placed at an angle to drain, and finished by washing it in water. Except for the purpose of after-etching, M. Niépce's process was of little commercial value then, but it has since been of some service in the practice of photo-lithography. That, I think, is the fullest extent of the commercial or artistic advantages derived from the utmost success of M. Niépce's discoveries; but what he considered his failures, the fact that he employed copper plates coated with silver for his heliographic tablets, and endeavoured to darken the clean or clear parts of the silvered plates with the fumes of iodine for the sake of contrast only, may be safely accepted as the foundation of Daguerre's ultimate success in discovering the extremely beautiful and workable process known as the Daguerreotype. M. Niépce appears to have done very little more towards perfecting the heliographic process after joining Daguerre; but the latter effected some improvements, and substituted for the bitumen of Judea the residuum obtained by evaporating the essential oil of lavender, without, however, attaining any important advance in that direction. After the death of M. Nicéphore Niépce, a new agreement was entered into by his son, M. Isidore Niépce, and M. Daguerre, and we must leave those two experimentalists pursuing their discoveries in France while we return to England to pick up the chronological links that unite the history of this wonderful discovery with the time that it was abandoned by Wedgwood and Davy, and the period of its startling and brilliant realization.