PLATE 42. 



Diatom. 



Pleurosigma angulatam x 5000. Mounted in realgar. 



Objective Zeiss 2 mm. Apo. oil immersion. N-.A. 1.30. 



Ocular Zeiss Gomp. 12. 



Condenser ... Watson's Holos. oil immersion. N. A. 0.80. 

 Illuminant ... 4 amp. arc Liliput. 



Aux, Condenser Double convex lens, giving parallel beam. 

 Screen ... ... Wratten's "B" (green). 



Plate Imperial N.F. 



Camera Ext. ... 850mm. 

 Exposure 10 sec. 



The resolution shown here is called the "white dot"; by a slight 

 alteration of focus the dots appear black on a lighter ground. 



As test object. An objective of low N.A., less than about N.A. 0.6, 

 shows no secondary structure, or gives a resolution into crossing 

 lines. The sharpness with which the white or black dots are revealed 

 by an oil immersion objective is a good criterion of its quality. 



An amplification of 5000 is rather unduly stretching the mag- 

 nification obtainable directly from an objective. 



Photo. W. B. R. I To follow Plate 41 and page 256. 



