PHOTOMICROGRAPHY OF DIATOMS 257 



Amphipleura pellucida (Plate 43, Figs. B and C) is 

 undoubtedly a fine test for accurate focussing ; to obtain 

 the transverse lines sharp and distinct requires patience 

 and nicety of adjustment of condenser and illuminant. 

 The lines can be discerned with axial illumination, but again 

 there is lack of sufficient contrast for photography, and 

 oblique light is required to obtain this. The slotted stop 

 (Fig. 50) should be placed in the condenser and arranged 

 so that light passing through the condenser impinges 

 on the diatom in the direction of its long axis (p. 178). 

 By carefully adjusting the stop a position will be found 

 in which the transverse lines will be seen to the best 

 advantage. Longitudinal striae can sometimes be made 

 out when the slot in the stop is at right angles to the long 

 axis of the frustule, and it is even possible to resolve the 

 striae into dots with the stop placed midway between 

 these two positions ; but this requires great skill and 

 depends to a great extent on the specimen itself, and very 

 few permit of a distinct resolution into dots, although 

 some present a distinctly beaded appearance in the 

 striae. 



Exposure and development are important factors in the 

 success or otherwise of a diatom photomicrograph ; too 

 short exposure results in a hopelessly thin negative, and 

 the flatness produced by over-exposure is equally 

 unsatisfactory. A developer giving good density and 

 contrast should always be used. Pyro-soda and hydro- 

 quinone with a moderate amount of bromide are the 

 most satisfactory. Gaslight papers give the most con- 

 trasty prints, but for fine detail glossy P.O.P. is some- 

 times better. 



Plant and Animal Sections (Plates 5, 6 and 11, etc.). 

 Very little need be said about the photomicrography of 

 plant and animal sections, as the notes accompanying the 

 plates call attention to the chief points. An objective 

 with a flat field is essential, and a large N.A. is very rarely 



