182 PHOTOMICROGRAPHY 



Dark Ground Illumination for Loiv Powers. There are 

 a great number of objects that may be most beautifully 

 displayed by this method with a central stop ; many 

 insects, diatoms, polyzoa, polycystina, etc., will be found 

 admirably suited for photography. In setting up the 

 microscope, carefully centre the light and condenser, 

 place the object in position and focus, insert the special 

 stop in its carrier below the condenser, and examine the 

 result on the ground glass : rack the condenser up if the 

 best result is not obtained. If the background is not 

 sufficiently dark try a larger stop or shut down the iris 

 of the Davis diaphragm until the best result is produced. 

 Colour fringes are frequently produced around the object, 

 as imperfections in the optical system are made most 

 apparent. If they occur a sharp negative is sometimes 

 impossible unless a monochromatic screen is used. In other 

 cases the production of interference colours gives a most 

 beautiful effect without any lack of sharpness in the 

 negative. Plate 22 shows the appearance often presented 

 by diatoms under these conditions of illumination. It 

 is essential that the slide be scrupulously clean, as any 

 particles of dust, scratches, etc., will show up as brilliant 

 points and lines of light, or as diffraction effects, and ruin 

 the negative. 



The photomicrographs on Plates 23 and 24 are given to 

 illustrate the use of dark ground illumination for dis- 

 playing some of the smaller forms of natural objects 

 which come within the scope of low-power work. 



Dark Ground Illumination for High Powers. The 

 method just described is unsatisfactory for high powers 

 unless a condenser of the most perfect chromatic and 

 spherical corrections is employed, and Leitz has recently 

 introduced a more highly corrected modification of his 

 oil-immersion condenser with the special object in view 

 of providing an illuminator suitable for all purposes. 



Greater brilliance and more complete freedom from 



