REPRODUCTION OF ILLUSTRATIONS 



is no longer visible, being covered by the fine wires of 

 the screen ; yet the general effect remains the same. It 

 is evident, therefore, that the visible parts of the picture 

 actually seen are minute square spaces of various grades 

 of tones and colors, each one represented in size by a 

 mesh of the screen. The picture in this condition is 

 simply a mosaic made up of a number of small squares. 



If each one of these squares is. examined separately 

 it will be found that the tone of each is practically uni- 

 form, sometimes slightly darker on one side or the other 

 as the tone of the picture grades from dark to light. 

 This surface, then, is one of dots the kind of a surface 

 necessary for reproduction by the direct photo-engrav- 

 ing process just described. 



Observing this and similar phenomena it seems to 

 have occurred to several engravers, shortly after the dis- 

 covery of the direct process of engraving, to attempt to 

 produce such an effect with very minute dots of a fine- 

 mesh screen upon the negative to be engraved. And 

 this was finally brought to practical perfection by Dr. 

 Max Levy, of Philadelphia, who invented a machine 

 with which almost microscopic, but still uniform, 

 parallel lines could be ruled upon glass. 



If anyone will take the trouble to hold any half-tone 

 picture close to his eye he will observe, what may not 

 have been apparent at the ordinary reading distance, 

 that the picture is made up of innumerable small dots. 

 This is the effect of the screen, and the homogeneous 

 surface of color is really an aggregation of minute dots 

 of color a great mosaic, just as in the case of the picture 

 with the wire screen over it. These dots, so inconspicu- 



[207] 



