ELEMENTARY PHOTO-MICROGRAPHY. 



DIATOMS. 



PLATE V. 



Fig. 20. Pleurosigma angulatum. 

 Rapid plate. "A" eyepiece. 



j^ oil immersion at 2oin. 

 Acetylene light. Twelve minutes. 



Fig. 21. Diatom showing markings. Oblique illumination. 

 ^5 oil immersion at 2oin. Eyepiece " A." Rapid plate. Ten minutes. 



