OBLIQUE ILLUMINATION 177 



which gives much greater resolution than can be obtained 

 by ordinary light. 



The Continental substage is particularly well-adapted to 

 give oblique light in any direction that may "be desired, 

 by racking the iris diaphragm to the edge of the ccn- 

 denser and rotating it until illumination is in the desired 

 azimuth. The control over the aperture of the diaphragm 

 and over its position is so complete that the best adjust- 

 ment can be obtained while the object is actually under 

 examination. Patch stops of various shapes, two of the 

 most useful of which are shown at Fig. 50, are required 

 to give oblique illumination with the English substage. 

 They are dropped into the ring and rotated until the 

 aperture is in the desired position ; or the direction of the 



Fig. 50 



STOPS FOU OBLIQUE ILLUMINATION 



light in reference to the object is adjusted by rotating 

 the stage instead. 



The photomicrograph of Amphipleura pellucida (Plate 

 43) may be cited as an example of the use of oblique 

 light. When examined with a wide-angle cone of direct 

 light, and an oil-immersion objective, this diatom is 

 resolved into parallel lines at right angles to its length. 

 It is difficult, however, to obtain a photograph in this 

 way, but if one of the stops, preferably the second, shown 

 in Fig. 50, is placed in the substage ring with its aperture 

 in what may be conveniently described as the " north " or 

 " south," while the diatom lies " north and south," these 

 transverse lines are made more distinct, and the photo- 

 micrograph can then be taken with comparative facility, 

 although even then a considerable amount of skill is 



