E. M. NELSON ON FINE MARKINGS ON NAVICULA MA JOE. 145 



account oblique light fails ; it is, therefore, exclusively a central 

 light test, and one of the best, for the minute points form a test for 

 the widest angled objectives of a modern microscope in its highest 

 development, and the transverse stride one for that most useful lens, 

 the Zeiss apo. ^ N. A. - 65, as well as one for cheap semi-apochromatic 

 quarters on second-class instruments, central light being employed 

 in both cases. 



This diatom can be readily procured, as it is common in 

 " Sozodont " tooth powder ; it should be mounted in quinidine, and 

 Mr. Gilford's latest F line screen should be used. 



