529 



NOTE. 



Photograph of Pleurodynia angulatum by F. E. Ives. 



Plate XII. 



At the June Meeting were exhibited a photograph of Pleurosigma 

 angulatum and lantern slides of Coscinodiscus aster omphalus, 

 Surirella gemma, and Pleurosigma angulatum. With regard to 

 these exhibits, which are not only beautiful in themselves, but 

 important from an optical point of view, Mr. Ives writes as 

 follows : — " This negative (referring to Pleurosigma angulatum) 

 is an enlargement from a smaller one made direct. The original 

 negative was made on a lantern slide plate, with Zeiss 3 mm. 

 apochromatic objective, N.A. 1*30, 18 compensation eye-piece, 

 Abbe achromatic condenser, on a Swift portable histological stand, 

 Welsbach light in ground-glass chimney, and the camera described 

 on pp. 371-8 of the Journal of the Franklin Institute, May 1902. 

 The mount is one of Moller's, dry, the valve in cover-glass con- 

 tact. Illumination, cone of about *80 or -90 1ST. A., very carefully 

 centered. Tube-length adjusted with great care. The objective 

 is marked for a tube-length of 160 mm., but when adjusted most 

 •carefully by the eye, on objects mounted in cover-glass contact or 

 in balsam, always stands within 2 mm. of 150. Quite incapable 

 of giving the result shown in this photograph with a 160 mm. 

 <tube. 



Direct amplification x 1320 ; enlargement to X 2375. 



With first-class achromatics, and with valves mounted in styrax 

 ■or monobromide, Pleurosigma angulation presents to my eye the 

 appearance shown in this photograph, and is so reproduced by 

 photography, though not quite so perfectly and brilliantly as with 

 the dry mount and apochromatic objective. 



The lantern slides of Coscinodiscus, Aster omplialus, and 

 ■Surirella gemma are contact prints from negatives made under 

 the same kind of conditions, from valves mounted in liquid- 

 amber. The Surirella shows, on one part, a suggestion of the 

 white dot growing up within the black dot, which is so well shown 

 in the photograph of Pleurosigma angulatum!' 



