JOURN. R. MICR. SOC. 1907. PI. I. 



o 



o 



o 



(mounted iu balsam). Photographed by 



§.A. = 0-35/ (N.A. = 0-35). 



Magnification 320 x 14 = 



Fig. 1. — Pleurusig))iw funnunniu 

 means of a narrow beam. 

 4480. 



Fig. 2. - Same sjjecimen, photographed by means of a broad beam, from which 

 the centre is stopped out by means of a stop, of which S.A. = 0'35/. The con- 

 denser beam is too much broken up in transmission by the diatom to be 

 measured, but the W.A. appears to be about 0-8/. The objective used has 

 S.A. = /; (N.A. = 1). Thus, the image of Fig. 2 is comx^lementary to that of Fig. 1. 



Fig. 3. — Same specimen, photographed by the same beam as Fig. 2, but with- 

 out the stop. The image is visibly a blend of the images of Figs. 1 and 2. 



