2 Lees, On the Superposing of Two Cross-line Screens 



screen, peculiar effects may be produced in the resulting negative. 

 The effects depend on the angle which the lines of the cross-line screen 

 make with the lines of dots (or other form pattern) of the original 

 picture. If this angle be small, the effect on the negative is to produce 

 in the reproduction a very coarse pattern of squares or other formation, 

 and this may be so obtrusive as to be more readily observed than the 

 subject. 



§3. We shall now consider the effect of photographing through an 

 intaglio screen a half-tone print, taken as consisting of a series of small 

 square dots surrounded by white lines arranged in square pattern. 

 The effect of the intaglio screen will be to produce in the final picture 

 (after the copper plate has been made and used to make a print) a 

 fresh set of white lines or bands forming a pattern of squares, and it 

 is the resulting effect of this set superposed on the set due to the 

 original half-tone print which brings about the result mentioned in 

 the preceding paragraph. It is clear that what we have to analyse, in 

 effect, is the result of superposing two intaglio screens, i.e., screens of 

 transparent bands with dark square spaces ; for only those portions of 

 the white bands of the original half-tone picture can affect a photo- 

 graphic plate, if rays of light passing from them penetrate through the 

 transparent bands of the intaglio screen. We shall therefore first 

 discuss the patterns produced by superposing two intaglio screens of 

 different rulings or pitches, and holding the pair up to the light. The 

 geometrical problem involved is best treated in the following way : — 



Let the two screens be denoted by S^ and S^. We may consider 

 .9, as formed by making two sets (J^i and F,) of parallel transparent 

 bands on a blackened plate, the two sets being at right angles to one 

 another, and arranged to produce a series of dark squares. Similarly 

 S^ may be considered as formed by two sets (A^, and K,) of transparent 

 bands on a second blackened plate, the rulings of these sets not 

 necessarily being the same as those of the first screen. We require 

 the final pattern R of transparent spaces obtained by superposing 5, 

 and S.., and holding the double screen up to the light. This pattern 

 R can be obtained by the following operations : — 



(/) Make two screens ruled respectively, one with set A'j only of 

 transparent bands, the other with set X^ only. Superpose these screens 

 and find the pattern P, of transparent spaces obtained on holding the 

 double screen to the light. 



{ii) Repeat operations (/) employing two screens ruled respectively 

 with sets Fi and K only. Let the resulting pattern of transparent 

 spaces obtained on holding the double screen to the light be denoted 

 by (2. 



{Hi) Repeat operations described above, using sets A'l and \\. Let 

 the resulting pattern be denoted by Pj- 



