542 Prof. Trowbridge and Mr. Crandall on Groove-Ft 



this is a numerical constant which can be obtained in 

 no other way, and which is essential in computing the 

 retardation over a single element o£ the groove. By making 

 a composite triangle, adding the homologous sides of six 

 grooves of an image of the microphotograph projected on a 



Fisr. 4. 



__ , A 



Fig. 4h 



\ sa &--9£iil cm. 

 4 fc> cos 2? 



fig 4c 



screen^ the dimensions of the groove (as sectioned) are found 

 as given in fig. 4 a. (The microphotograph is shown on the 



