256 PHOTOMICROGRAPHY 



concentric lines, due to diffraction, surrounding the diatom 

 can be easily made out on the right half of the figure, and 

 there are also round marks on the background due to 

 minute specks of dust which have been brought into 

 prominence by the narrower cone of light passing through 

 the eyepiece. 



Pleurosigma angulatum (Plate 41, Fig. C, and Plate 

 42) is one of the most difficult of diatoms to photo- 

 graph. The secondary markings can be made out either 

 as white dots, circular apertures or hexagons, or, by a 

 slight alteration of focus, as black dots with a white 

 surround. There is a diversity of opinion as to which 

 is the correct focus ; the white dot is undoubtedly the 

 easier to photograph. We give two illustrations of the 

 diatom, both taken with axial illumination ; one at medium 

 magnification (Plate 41, Fig. C) was taken with a 

 student's microscope and home-made box camera, merely 

 to show that, if care is taken, satisfactory photographs 

 can be obtained with the simplest form of apparatus 

 possessing none of the refinements and accessories that 

 are often regarded as absolutely necessary when using 

 immersion objectives. 



The other illustration (Plate 42) is of very high 

 magnification, viz. 5000 diameters. It was obtained by 

 using a T V" immersion objective together with a high 

 eyepiece and long camera extension ; but it is doubtful 

 if any advantage is to be obtained by carrying direct 

 magnification to such an extent, and in many cases a 

 sharper print can be obtained by enlargement from a nega- 

 tive taken at 1000 or 1500 diameters (cp. Plate 43, Fig. C). 



Surirella gemma (Plate 43, Fig. A). It is difficult to 

 resolve the secondary markings in this diatom unless 

 oblique light is used. If the specimen is mounted in 

 realgar the dots can be seen with axial light, but there is 

 not sufficient contrast to obtain a satisfactory photo- 

 micrograph, hence the need of oblique illumination. 



