The development of the latent image by mercury subliming was the most marvellous and unlooked-for part of the process, and it was for that all-important thing that Daguerre was entirely indebted to chance. Having put one of his apparently useless iodized and exposed silver plates into a cupboard containing a pot of mercury, Daguerre was greatly surprised, on visiting the cupboard some time afterwards, to find the blank looking plate converted into a visible picture. Other plates were iodized and exposed and placed in the cupboard, and the same mysterious process of development was repeated, and it was not until this thing and the other thing had been removed and replaced over and over again, that Daguerre became aware that quicksilver, an article that had been used for making mirrors and reflecting images for years, was the developer of the invisible image. It was indeed a most marvellous and unexpected result. Daguerre had devoted years of labour and made numberless experiments to obtain a transcript of nature drawn by her own hand, but all his studied efforts and weary hours of labour had only resulted in repeated failures and disappointments, and it appeared that Nature herself had grown weary of his bungling, and resolved to show him the way. The realization of his hopes was more accidental than inferential. The compounds with which he worked, neither produced a visible nor a latent image capable of being developed with any of the chemicals with which he was experimenting. At last accident rendered him more service than reasoning, and occult properties produced the effect his mental and inductive faculties failed to accomplish; and here we observe the great difference between the two successful discoverers, Reade and Daguerre. At this stage of the discovery I ignore Talbot's claim in _toto_. Reade arrived at his results by reasoning, experiment, observation, and judiciously weakening and controlling the re-agent he commenced his researches with. He had the infinite pleasure and disappointment of seeing his first picture flash into existence, and disappear again almost instantly, but in that instant he saw the cause of his success and failure, and his inductive reasoning reduced his failure to success; whereas Daguerre _found_ his result, was puzzled, and utterly at a loss to account for it, and it was only by a process of blind-man's bluff in his chemical cupboard that he laid his hands on the precious pot of mercury that produced the visible image.