The complete process by which pictures are made by the chemical action of light on a sensitized plate or film is known as photography. Many chemicals are sensitive to and affected by light. A common example is – salt of silver chloride darkens by prolonged exposure to light.
To make a picture, first of all a chemically prepared surface is needed, which is generally a uniform coating over glass, celluloid or paper. An emulsion of insoluble silver halides and gelatin is prepared and the its coating is done on glass, celluloid or paper. All this has to be done in total darkness.
The camera is a wonderful little instrument for producing pictures. It is a light-tight box with a lens fitted in the front. There is an arrangement for keeping the sensitive surface i.e. plate or film at the back. For the light to reach the film, through the lens, there is a device known as the shutter. The purpose of the lens is to form an image of the object at the back within the camera, where the film is kept. The formed image is real but inverted. When the shutter is released for some small and appropriate duration of time, an exposure is said to have been given to the film. At this stage, if the film is examined, no image will be visible. However, a latent image would be formed on the emulsion coating on the film. When treated with developer, the film produces an image that is visible.
The process of developing the film and plates is conducted in total darkness. However, in a few cases, a very weak red light can be used. The developed image will be black and white. The parts which get more light become darker and those which get less light remain transparent. The reason is that silver salts are reduced to metallic black silver by the action of the developer and exposure to light. After developing, the film is kept in the fixer, which dissolves and removes all unexposed silver salts and those areas become transparent. This way a negative is prepared, which is just the reverse of the object. To get a correct reproduction of the object the negative is put in contact with photographic paper, which is also coated with an emulsion of silver salts and is exposed to white light through a printing box. On developing the exposed paper and fixing it, the object appears in its proper order. The final result is called a positive. Bigger and enlarged pictures also can be made from the same negative with the help of an apparatus known as Enlarger.
Photography was not discovered by any single person. As such, it was the result of constant research in the field of chemistry and physics. As far back as 14th century, someone happened to go to a darkened room in one of the doors in which there was a very small hole and rays of sunlight passing through this hole, forming a true but inverted image of the outside scene on the opposite wall of the darkened room. In the year 1569 Della Porta of Italy discovered the possibilities of using a glass lens in the place of the hole for a sharp projection of the image of the outside landscape.
As all discoveries go, this was again taken up and in 1686, Johann Zahn described in his journal the construction of a portable type of instrument with a lens and a mirror to present the image with the right side up ( on the same principles as the single lens reflex of today).
At this stage the problem was how to make this image permanent on some sort of surface like paper or glass. In the 18th century, Gaber found out that silver chloride become dark when exposed to white light, but nothing further was done in this direction till 1727, when G.H.Schulze made an emulsion of silver nitrate and chalk and coated it on a sheet of metal. Then he placed a transparent paper with some opaque words written on it in contact with the coated metal sheet and exposed it to the sunlight. The coated surface below the words was not exposed and was found white, while the surrounding areas turned black. In the year 1777, Charles William Scheele did some experiments with silver chloride under the influence of various colored lights. He discovered that red and yellow lights had very little effect but other lights completely darkened silver chloride. In 1780 silhouette pictures were made on a paper coated with silver salt solution, by keeping the head of a person in the beam of sunlight and sharp shadows thus created were received on the coated paper. In 1802 Thomas Wedgwood and Sir Humphry Davy also discovered similar phenomena.
Now the problem was to fix the image on the coated paper, and to save it from becoming black on exposure to light. In the late 18th century, Sir John Herschel discovered sodium thyosulphate as a fixing agent. In the year 1824 some pictures on glass and metal sheets were made by Joseph Nicephore of France. He coated the glass with a layer of bitumen dissolved in the oil of lavender and then after treatment with an acid he obtained a negative to produce a positive. It was not until 1839 when Mande Daguerre prepared a mixture of silver chloride and iodine and coated it on metal plate. Then he gave an exposure for about three hours in the camera but got a very faint image. One day, due to very dull light he kept his exposed plate after removing it from his camera in his chemical cabinet to reuse it. But next day he found that the image on the exposed plate was perfect and clearly visible but he could not understand how it happened. Later he found that it was due to vapours from a bowl of mercury. Thus the first developing agent was discovered by him. He published the details of his method on the request of the French Govt. and named it Daguerreotype. In 1841, W.H. Fox Talbot prepared for the first time a photography paper for making negatives and positives and named his process as Talbotype or Calotype. In the year 1849 , he prepared the glossy type paper also. In 1851, Scott Archer of London introduced his collodion wet plate method of making negatives in the camera. In 1871, Dr. R.L.Maddox invented the present dry plate, using gelatin in place of collodion. In 1898 Reverend Hannibal Goodwin of America introduced the Roll Films which were commercially prepared by Eastman Kodak Company for the first time.
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