}[a)icJicsti'r Mi-i>ioirs, I ^ol. Ixiii. ( 1 9 1 9) No. 4 



Plate III. 



Fig. la. 



Illustrates the effect of superposing " chess board " screens. Pitcli of screens 



100 to the inch. Angle aj^". (Actually, the screens consist of Vjlack round 



dots on a white ground.) 



.Same as in Fi». 7«. but angle is now 5' . 



