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击实试验的原理与压缩实验的原理一样都是土体受到压密。()
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According to the author, what may chiefly be responsible for the moral decline of Japanese
A．He runs after the ball.B．He changes positions.C．He throws the ball.D．He tries to hit
A．He was a brave soldier.B．He took war photographs.C．He painted portraits.D．He designe
听力原文： Today, I'll be talking about the invention of the camera and photography. The camera is often thought to be a modem invention. But as early as 1727, a German physicist discovered that light darkens silver salt, a chemical compound. Using as a camera, a big box with small hole to let the light in, he made temporary images on the salt. Silver salt is still the base of film today.
Then a French scientist made the first permanent picture by using a special piece of metal sensitized with silver salt. A photograph he made in 1826 still exists. The painter, Daguerre improved on the process by placing common salt on the metal, This was in 1839, the official date of the beginning of photography, but the problem was the printing of the photographs, and it wasn't until other scientists developed the kind of paper we now use, that good printing was possible and photograph became truly modern.
In 1860's, Matthew Bradey was able to take his famous pictures of the American Civil War. In the 20th century, George Eastman of the Untied States simplified film developing, and Dr. Edwin Land invented the so-called "instant" camera with self-developing film. If we say that photography came into existence in 1839 it follows that it has taken more than one hundred years for the camera to reach its present condition of technical refinement.
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