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



as little as we please from the original pattern (suitably displaced). In 

 this sense then, we can say that R depends only on 6- 



In concluding this Appendix, it may be mentioned that periodic 

 frameworks, of the kind we have been discussing, are well known in 

 the Theory of Numbers and in the Theory of Doubly Periodic 

 Functions, under the heading Zahlengitter. The author has not been 

 able to find any evidence that Zahlengitter have been discussed from 

 the optical point of view. 



