HALFTONE SCREEN — KAINEN 411 



effects by engraving on the end-grain of a hard, dense wood, such as 

 boxwood. With this innovation w^ood engraving came into univer- 

 sal use for relief printing, reaching its culmination in the last quarter 

 of the nineteenth century. At tliat time, the demand for pictorial 

 matter in books, magazines, and newspapers reached higher levels than 

 ever before. It had been increased by developments in photogravure 

 and collotype — and in lithography, which Alois Senefelder discovered 

 in Germany in 1798. This process made use of slabs of Bavarian 

 limestone, which had the property of being sensitive to both grease 

 and water. The drawing was made on the stone with a crayon or ink 

 containing grease and pigment and chemically treated to fix the greasy 

 image and desensitize the remainder of the stone to grease. The stone 

 was then covered with a thin film of water, which was rejected by 

 the greasy image but retained by the porous stone. A greasy ink was 

 applied by a roller and was accepted by the greasy image but rejected 

 by the damp areas. The original drawing was therefore reconstituted 

 in printers' ink and was susceptible of printing under a slight scrap- 

 ing pressure. Lithography proved to be a versatile process that of- 

 fered the simplest and most direct method for printing toned pictures 

 before the advent of photomechanical printing. 



The modern halftone would not have been possible, of course, with- 

 out the invention of photography. In 1839 Daguerre, in Paris, con- 

 tinuing the work of Niepce, announced the invention of the daguerreo- 

 type. A little later in the same year Fox Talbot in England reported 

 on the calotype. Of the two inventions Daguerre's was the more im- 

 mediately successful, although Talbot's unquestionably was the more 

 important in the long run, since it introduced the use of transparent 

 negatives. The daguerreotype produced only a single final silver 

 image on a copper plate. Numerous experimenters immediately be- 

 gan to etch daguerreotype plates and to use electrodeposition in an 

 effort to turn them into printable surfaces. Although these attempts 

 produced interesting results they were basically unsatisfactory and 

 were soon abandoned. 



In 1852 Fox Talbot made another contribution of fundamental im- 

 portance. It was known, from the observations of Ponton and Bec- 

 querel, that gelatin, when sensitized with a bichromate salt, has a 

 propensity to harden under the action of light. Talbot was the first 

 to make practical use of this phenomenon in patenting the photo- 

 gravure process, or, as he called it, "photoglyphic engraving." His 

 1852 patent, at the same time, laid the basis for practically all future 

 developments in printing from a photographic image. 



The earliest form of the Talbot process involved coating a steel 

 plate with a mixture of gelatin and bichromate of potash, exposing 

 it under a positive, washing away the unhardened gelatin, and etchmg 



