1614  Angelo Sala noted that silver nitrate turned black when exposed to the sun, but he apparently saw nothing significant in the change and did not ascribe it to the action of light.

1725  The first discovery of importance was made by Johann Heinrich Schulze, a professor of anatomy at the University of Altdorf. He had mixed powdered chalk into a solution of nitric acid in an attempt to make a phosphorescent material (the aluminous stone"   of the alchemist, Balduin) and was amazed to discover that the   mixture turned dark violet in sunlight. He traced the discoloration to a contaminant in the acid, silver, and eventually proved that   silver compounds were visibly changed by the action of light rather than heat or exposure to air, as had been previously  suggested. Schulze made numerous stencil prints on the sensitive contents of his bottles, but apparently he never applied the  solutions to paper or made any attempt to record natural images.

1777  Carl Wilhelm Scheele, the Swedish chemist, investigated the properties of silver chloride and made some interesting discoveries. Like Schulze, he established that the blackening effect on his silver salt was due to light, not heat. He also proved that the black material was metallic silver and he noted that ammonia, which was known to dissolve silver chloride, did not affect the blackened silver. If Scheele had realized the importance of this last   discovery, he could very well have become the inventor of photography because by this time the essential processes were known.

1795  This essential idea finally came to Thomas Wedgwood, the youngest son of the famous potter, Josiah. In addition to being an outstanding craftsman and artist, Josiah was a brilliant and respected member of the English scientific community. Thomas was familiar with the camera obscura because his father had used it as an aid in drawing scenes for use on his pottery. The Wedgwood family also owned the notebooks of William Lewis, who in 1763 had described Schulze's and his own experiments with the silver compounds. These circumstances and natural curiosity prompted young Thomas to begin experiments of his own, probably about 1795. Thomas Wedgwood narrowly missed becoming the inventor of photography for two reasons. He gave up attempts to make pictures with the camera obscura (his exposures were not sufficient), and he was unable to fix the silver images he did produce by direct printing.

1816  In France, meanwhile, Joseph Nicephore Niepce and his son Isidore were busy experimenting with lithography at the family estate near Chalon. When Isidore, who had been copying drawings onto the stones for his father, joined the army, and Nicephore began to explore lightsensitive varnishes, hoping to find a coating for the stones that would record the drawings by exposure to light. He must have made some progress because in 1816 he set out to take pictures from nature using a camera and paper sensitized with silver chloride.
Niepce had limited success almost immediately, but he was  displeased because the image tones were reversed from nature (they were negative) and he could not make the image permanent. He realize that the tonal reversal was an inherent part of the silver  process and tried to produce a positive print by reprinting one of his negatives, but his attempts were unsuccessful. He also found that nitric acid helped to preserve the image for a while, but it only   postponed disaster and could not prevent it. He began to experiment with other materials.

1822  Finally, in 1822 he (Niepce) produced a copy of an engraving by exposing through the original onto a glass plate coated with  bitumen of Judea, a type of asphalt. Light hardens this material, so when Niepce washed his exposed plate with the usual solvents, only the unexposed portions were floated away, leaving the image in permanent lines. He called his process heliography (sunwriting). He made a number of similar heliographs in the next few years and continued his efforts to record a camera image. At last, in 1826, he succeeded. The world's first permanent camera image shows the view from Niepce's second floor window and is little more than an impression. It is a bitumen image on pewter, showing only masses of light and dark tones. The exposure  supposedly took about eight hours.

1835  Louis Jacques Mande Daguerre discovered (quite by accident, if the story is true) that treatment with mercury vapor would produce a visible image on an iodized silver plate that had been briefly exposed to light. He also managed to stabilize the image with astrong solution of salt.
  In 1838 he contacted a group of leading French scientists, among them Francois Arago, and solicited their help. Arago was  immediately impressed with the invention and made a brief announcement of it at the Academie des Sciences in January 1839.

August 1839 Arago succeeded in convincing the government that French national honor was at stake. A bill was passed granting life pensions to Daguerre and Niepce, and the details of the process were announced to a frenzied public in August 1839. Although the French government had announced that the process was now public   property, this was not entirely true. Daguerre had secretly patented it in England just a few days before the formal French  announcement.
News of the daguerreotype process spread like wildfire.  Enthusiastic experimenters, French and foreign, were soon happily engrossed in the new technique, but there was dismay in England.  A respected member of the Royal Society of London, William   Henry Fox Talbot, saw the new process as a threat to his own investigations. In an attempt to establish priority, Talbot had written Arago on January 29,1839, claiming that he had been the first to find a method for taking pictures with the camera obscura and for fixing them. He overstated his case. He had not accomplished much more than Niepce had in his experiments with silver chloride, and his method of fixation was far from satisfactory.
 
Timeline of camera-related discoveries
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© Werner Hammerstingl 1998