SCIENCE IN THE INDUSTRIAL WORLD 



ous but still the substance of the picture, are made by 

 the glass screen which is the all-important agent in half- 

 tone reproduction processes. 



This screen is made of two pieces of glass, ruled with 

 minute parallel lines, so placed that they cross at right 

 angles, giving the mesh-like appearance of the screen. 

 To prepare such a screen, the surface of a piece of glass 

 is coated with some substance analogous to the coating 

 used on the surface of the metal plate by an etcher. 

 This surface is then ruled by delicate machinery in 

 minute parallel lines, the diamond ruling-point of the 

 machine removing a minute line of the coating, but 

 leaving a corresponding ridge of it between each line. 

 These lines are at mathematically equal distances apart, 

 and there are from about fifty to as many as four hun- 

 dred to the inch. When ruled, the surface of the glass 

 is treated to an acid bath, which eats out the surface 

 of the glass in the paths made by the diamond point. 

 When sufficiently etched the glass is cleaned, and an 

 opaque pigment rubbed into the rulings. Another 

 plate, treated in a similar manner but with the rulings 

 running at right angles, is now fitted to the surface of the 

 first one, the result being the checkered appearance seen 

 in the half-tone print. 



To reproduce a picture by means of this screen, an 

 ordinary negative is made just as in direct photography, 

 except that the screen is interposed between it and the 

 picture or object to be photographed. The resulting 

 negative, which is called the "screen negative," shows 

 the minute meshes on the screen, just as observed in the 

 finished print. 



[208] 



