PATTERN-DISCRIMINATION IN VERTEBRATES 



329 



The instrument reached its present form by a number of 

 accretions. In 1906 Behn 8 called attention to certain properties 



JX 



n 3 . 2 



!♦♦♦♦♦ 



iiii 



mm 



figs. 1 and 2; showing principle of crossed gratings (after Behn). 



of superposed gratings. He says: "The points of intersection 

 of two sys tems (I and II, Fig. 1) of parallel equidistant straight 



8 Behn, U, " Ueber eine neue Dilatometer Prinzip fur Projection," Berichte d. 

 Deutschen Physikalischen Gesellschaft, IV, 1906, S 207 ff. 



