178 



PHOTOMICROGRAPHY 



required to get a good result. If the stop is then rotated 

 until its aperture is in the " east " or " west," longi- 

 tudinal lines appear, while if it is in the " north-west," 

 " south-east," etc., resolution into dots instead of lines 

 is sometimes obtained. The procedure just described is 

 that always employed with oblique illumination, the 

 beam of light is caused to fall perpendicularly on to the 

 direction of the lines to be resolved or emphasised. 



Fig. 51 



RESOLUTION BY OBLIQUE LIGHT 



Fig. 51 illustrates diagrammatically the procedure 

 adopted to resolve the details of such diatoms as Am- 

 phipleura pellucida or Navicula rhomboides. Navicula is 

 the simpler, and the resolution can usually be effected 

 by a good J" objective. The arrows show the direction 

 of the oblique light necessary to resolve the diatom as 

 indicated. 



Amphipleura pellucida is often considered as a good 

 test object for oil-immersion objectives, but the resolution 

 into dots with oblique light depends more on the specimen 

 than on the objective, and, if a suitable specimen can be 

 obtained, an objective that cannot claim to be first-class 

 will resolve it, but such specimens are rare. Navicula 

 rhomboides and Surirella gemma can be resolved into 

 dots in axial light, but the dots appear with greater con- 

 trast when oblique is employed. 



