PLATE 24. 



Dark Ground Illumination. 



A B 



Lophopus crystallinus GROUP OF POLYCISTINA 



x 20 x 45 



Objective Crouch 2" Achromat. Leitz 3, 16 mm. 



Ocular Leitz 0. None. 



Condenser ... Spot lens. Abbe with wheel stop. 



Illuminant ... 4 amp. arc. Limelight. 



Aax. Condenser Double convex lens, giving parallel light. 



Plate Imperial Special Rapid. 



Exposure 6 sec. 2 sec. 



The spot lens referred to is a plano-convex lens of about 1" to 2" 

 focal length, with the top surface partially obliterated by a black 

 patch. 



A Davis nose-piece diaphragm was used for both photomicrographs 

 and closed down until the best result was obtained. 



Lophopus, one of the fresh-water Polyzoa, is an almost clear 

 gelatinous organism forming a very beautiful object for dark ground 

 illumination. The slide was mounted by Clarke '& Page in a special 

 fluid which, to a great extent, preserves the natural appearance of 

 the Polyps. In the photomicrograph several individuals are seen 

 e\ lending from the common body. 



Polvcistina are fossil siliceous skeletons of minute marine 

 unicellular organisms (Protozoa); by deposition as Radiolariari oozes 

 and ultimate consolidation they have built up rock masses in 

 Barbadoes and other parts of the world. 



Photos. W. B. ft. \Tofollowpage182. 



