!0 Lees, On the Superposing of Tivo Cross-line Screens 



APPENDIX. 



The effect of such displacements, as are referred to in >;2o, can be 

 discussed analytically as follows (see Fig. 13) : — 



Let the centre of a black square of the screen .Si be denoted by O, 

 and through O take two axes Ox, Oy parallel to the sides of the square 

 (and therefore mutually at right angles). After superposing screen 6*2 



Fig. 13. 



on screen S„ let the centre 6>' of a black sfjuare of 5' have the 

 co-ordinates («, (i) relative to Ox, Oy. Through O' draw axes O'l, O'ti 

 parallel to the sides of the squares of S\ These axes will be mutually 

 at right angles, and O'E can be taken as making the angle with Ox. 

 For simplicity in the following discussion, we shall put 



«i + ^i = yi 

 a 



+ ^i = 7i, \ 



(0 



