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On the Resolution op A. pellucida and a Violet Copper- 

 Iodine Light Filter. 



By Prof. Zettnow, of Berlin. 

 CCommunicated hy G. C. Karop, May 19th, 189S.J 



Of all microscopic test-objects A. pellucida is by far the 

 most difficult : whilst Van Heurckia crassinervis and Surirella 

 gemma can be shown completely resolved with good objectives 

 and central, or but slightly oblique illumination and daylight, 

 the same glasses under the same conditions only give the 

 mere outlines of Amphipleura, and only then provided the 

 diatom is mounted in realgar with a high refractive index 

 of 2-4. 



One only succeeds in rendering the transverse stride visible 

 even with strongly oblique illumination, whilst the much more 

 delicate and closer longitudinal lines demand the most ex- 

 tremely oblique light, and, therefore, small aperture, so that 

 numerous and well-marked diffraction lines are unavoidable. 



By using instead of white light the dark-blue, transmitted 

 by cupric-ammonia solution, the resolution becomes easier, as 

 an objective of certain aperture is capable of resolving even 

 finer series of lines as the wave length of the light employed 

 is shortened, and hence the employment of a blue light filter 

 in photographing diatoms. The numerical aperture of an 

 apochromatic of 1"4 in daylight rises to 1-65 if dark-blue 

 light is used. Following this train of thought, I endeavoured 

 to increase the performance of a Zeiss apo. of 1"4 N.A. by 

 employing a violet filter, which permitted the use of rays of 

 lesser wave length than a dark-blue, not merely to resolve the 

 transverse and longitudinal strise of the Amphipleura, each 

 with its own most favourable illumination, but to photograph 

 them both at the same time ; in other words, to get the 

 Amphipleura resolved into beads such as are exhibited by the 

 easier test-objects. The diffraction lines, which when they 



