tailieunhanh - An Encyclopedia of the History of Technology part 76

An Encyclopedia of the History of Technology part 76. This one of a kind encyclopedia presents the entire field of technology from rudimentary agricultural tools to communication satellites in this first of its kind reference source. Following an introduction that discusses basic tools, devices, and mechanisms, the chapters are grouped into five parts that provide detailed information on materials, power and engineering, transportation, communication and calculation, and technology and society, revealing how different technologies have together evolved to produce enormous changes in the course of history | PART FOUR COMMUNICATION AND CALCULATION Judaea dissolved in animal oil coated on to sheet of glass copper or pewter. He made a sandwich of engraving sensitized plate and a sheet of glass which he exposed to the sun for two to four hours which hardened the exposed varnish. Then in a darkroom he immersed the plate in an acid bath to dissolve the varnish protected by the lines of the engraving the remaining hardened varnish formed the image. He called this process as well as the images he made in a camera obscura heliography. The earliest known such image is a view of neighbouring rooftops made by exposing a pewter plate in a camera obscura for eight hours because of this extremely lengthy exposure of the first true photograph the sun can be seen lighting both sides of the rooftops. Niepce soon changed his material to silver-plated copper and gave the plates extra sensitivity by iodine fuming. He also tried glass plates viewing them as transparencies so it is surprising that he did not realize he could use these as negatives to make multiple prints the basis of most modern photography. This invention was not made until 1835 by Henry Fox Talbot in England. In a camera obscura he exposed a sheet of paper which had been sensitized with silver chloride. The first such negative he made was of a window in his home Lacock Abbey in Wiltshire. Unlike the plates of Niepce Fox Talbot s process required an exposure of only thirty minutes. SirJohn Herschel who named the invention photography and also coined the terms negative and positive suggested removing the unexposed silver chloride to fix the image permanently using hyposulphite of soda. With its name shortened to hypo this is still the standard fixing solution. In 1840 Talbot discovered as had Daguerre two years before that an exposure of only a few minutes could provide a latent image which could be developed by chemical means afterwards. Fox Talbot used gallic acid as a developer and after drying the paper negative made it

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