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Transactions of the Society. 
the individual objective used. Too narrow a cone is apt to cause 
diffraction fringes, too wide a cone produces haze. I have not had 
much experience in photographing test diatoms, hut so far as it goes, 
I find that my own 2 mm. Zeiss Apochromatic, 1*4 N.A., gives its 
best definition of such objects when about two-thirds of its back lens 
is filled by the dioptric beam. 
I trust this description of my apparatus will enable others who 
may be desirous of using sunlight illumination to adopt it, and, I 
hope, improve upon it. I shall be pleased to answer any inquiries 
as to any point that may not have been made sufficiently clear. 
