In 1826, a Frenchman named Niepce needed pictures for his business．But he was not a good artist．So he invented a very simple camera．He put it in a window of his house and took a picture of his yard．That was the first photograph．

　　The next important date in the history of photography was 1837．That year, Daguerre, another Frenchman, took a picture of his workroom．He used a new kind of camera and a different process(程序)．In his pictures, you could see everything very clearly, even the smallest details．This kind of photograph was called daguerreotype(银版照相)．

　　Soon, other people began to use Daguerre’s process．Travelers brought back daguerreotypes from all around the world．People photographed famous buildings, cities and mountains．

　　In about 1840, the process was improved．Then photographers could take pictures of people and moving things．The process was not simple．The photographers had to carry lots of film and processing equipment．But this did not stop the photographers, especially in the United States．After 1840, daguerreotype artists were popular in most cities．

　　Matthew Brady was one well-known American photographer．He took many photos of famous people．The photos were unusual because they were very life-like and full of personality．

　　Brady was also the first person to take pictures of a war．His 1862 Civil War pictures showed dead soldiers and destroyed cities．They made the war seem more real and more terrible．

　　In 1861, Scottish physicist James Maxwell showed a color photography system involving three black and white photographs, each taken through a red, green, or blue filter(滤光镜)．This is the “color separation”method．

　　In the 1880s, new inventions began to change photography．Photographers could buy film ready-made in rolls．So they did not have to make the film themselves．Also, they did not have to process the film immediately．They could bring it back to their workrooms and develop it later, meaning they did not have to carry lots of equipment．And finally, the invention of the small hand-held camera made photography less expensive．

　　With the small camera, anyone could be a photographer．People began to use cameras just for fun．They took pictures of their families, friends and favorite places．They called these pictures“snapshots．”

　　Photographs because very popular in newspapers in the 1890s．Soon magazines and books also used documentary photographs．These pictures showed true events and people．They were much more real than drawing．

　　Photography also turned into a form of art by the end of the 19th century．Some photographs were not just copies of the real world．They showed ideas and feelings, like other art forms．

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　　David’s got a text message from his friend Sam．Can you understand it?

　　7,15　　20,15　　20,8,5　　16,1,18,11

　　This message was in code(密码，代码)．How many letters are there in the English alphabet?Twenty-six．So can you read it now?

　　David decided to use a different code in his answer to Sam．This time, he was putting A instead of B, B instead of C, and so on．His message to Sam shown on the right was very short．Can you understand?

　　Sam guessed David’s code, and he replied：ZESDQ RBGNNK．When David’s little sister saw the message, she felt puzzled．She couldn’t read it at all．But David was very happy that his sister couldn’t read his message!

　　Lots of people use codes to send secret messages．In a war, secrets are very important so soldiers often send each other messages in code．Nowadays, we can use computers to make and break difficult codes．Even before computers, people invented other machines to do this．

　　Here’s a famous machine that Germans used in the Second World War(1941－1945)．It’s called the Enigma Machine．The machines were very big, heavy and expensive．Every day they changed the codes automatically, and this made it almost impossible for anyone to break the codes．The person sending a message had one machine, and the person receiving the message had another．

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