Manchester Memoirs^ Vol. Ixiii. (1919) No. 4 



IV. On the Superposing of Two Cross-line Screens at Small 

 Angles and the Patterns obtained thereby. 



By S. Lees, M.A. 



(Read and received April ist, igiQ. ) 



§1. The following investigations were originally undertaken by the 

 author at the suggestion of Mr. R. B. Fishenden, Director of the 

 Printing Department, College of Technology, Manchester. 



§2. In certain technical processes it is necessary to superpose two 

 cross-line screens or other screens possessing a regular geometrical 

 pattern. The result is to produce under certain conditions a character- 

 istic pattern, and it is the object of this discussion to explain some of 

 the phenomena produced. 



One of the most interesting examples of the effect occurs in the half- 

 tone process of photo-engraving. In this process, the picture to be 

 reproduced is photographed through a half-tone or " process " screen, 

 placed in front of the sensitive plate. The screen consists of two glass 

 plates ruled with parallel lines in black placed at right angles to one 

 another, so that the whole constitutes a series of equal sized small 

 squares, in effect. From the negative a photographic print is made on 

 to a copper plate which is afterwards etched. A print from the copper 

 plate will consist of a series of black dots which increase in size with 

 the depth of tone, until only white spaces remain.* 



Several different types of cross-line screens are used in technical 

 processes for photo-engraving ; for example, in the process of intaglio 

 machine photogravure a screen consisting of thin white lines on a 

 black ground is employed. In other cases a screen may be employed 

 which consists of a "chess board" pattern of black and white squares. 

 In this paper, the first type of screen will be described as a half-tone 

 screen, the second type as an intaglio screen, and the third type as a 

 chess board screen. 



It is sometimes necessary to reproduce a print from a half-tone 

 block or other picture consisting of dots possessing a regular geomet- 

 rical pattern. If such a picture be photographed through a cross-line 



* For technical details see a paper by Mr. Fishenden, Trans. Royal Photo- 

 graphic Society, March 15th, 1915. 



September jotk, igig. 



