PLATE 22. 



Direct Colour Photomicrograph. 



Screen-plate process (Dufay). 



GROUP OF DIATOMS x 56 



Dark ground illumination. 



Objective Leitz No. 3 (f") Achromatic. N.A. 0.30. 



Ocular None. 



Condense?- ... Abbe with wheel stop. 



Illuminant ... Arc 4 amp. 



Aux. Condenser Double convex lens, giving parallel light. 



Plate Dufay colour plate. 



Screen Special colour screen for arc light. 



Exposure 2min. 



The diatoms are colourless with transmitted light, but show varie-| 

 and delicate tints, due to interference phenomena, when examin 

 under dark ground illumination. 



Photo. W. B. R. 



Prep. W. A. FIRTH. \Tofollowpage 



