SCIENCE IN THE INDUSTRIAL WORLD 



pensive form of illustration they would be if they were 

 real etchings. 



This form of reproducing pictures, known as the 

 "direct " method, was soon succeeded by the discovery of 

 a method employing the same principle but which could 

 be used in reproducing such flat-toned pictures as the 

 photograph. This ingenious process, perhaps the 

 most wonderful as well as one of the simplest processes 

 of reproduction, is what is known as the "half-tone" 

 process, made so familiar in the last two decades by the 

 illustrated magazines of every description, in which 

 most of the pictures are made in this manner. 



This process differs from the foregoing in that a 

 " screen" is interposed between the picture and the nega- 

 tive in making the "screen negative" for printing on 

 the metal plate to be engraved. For this reason the 

 half-tone process is called the "indirect," in contra- 

 distinction to the "direct" one. The principle of 

 producing gradations in tone is accomplished by fine 

 dots or lines in the one, just as in the other, only these 

 dots are made artificially upon the negative and not on 

 the picture itself, and are so minute that they are not 

 noticeable except by careful scrutiny. 



If an ordinary piece of wire screen, such as is used on 

 the screen-doors in summer time, is placed in close con- 

 tact over a picture of any considerable size, it will be 

 observed that, while the picture is somewhat obscured, 

 it is still easily discernible; and if it is held at a little 

 distance it appears almost as distinct as without the 

 overlying screen, the meshes of the screen practically 

 disappearing. Obviously, a large surface of the picture 



[206], 



