A、special paper. B、silver salt. C、common salt. D、a special metal. D

Today I will be talking about the invention of the camera and photography.
   Camera is often thought to be a modern invention. But as early as 1727, a German physicist discovered light darkened silver salt, a chemical compound used as a camera, a big box with a small hole to let the light in, he made temporary images on the salt.  Silver salt is still the basic film today. Then a French scientist made the first permanent picture by using a special piece of metal sensitized with silver salt. A photograph photograph he made in 1826 still exists.
   The painter DeGair improved on the process by placing common salt, the kind we eat, 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 was not until other scientists developed the kind of paper we now use that good printing was possible, and photography became truly modern.
   In the 1860’s, Mathew Brady was able to take his famous pictures of the American Civil War, thus making portrait poses very popular. In the 20th century, George Eastman of the United States simplified film developing, and Edwin Land invented the so-called instant camera with self-developing film.
     If we say photography came into existence in 1839, it follows that it has taken more than 100 years for the camera to reach its present condition of technical refinement.