122 PHOTOMICEOGRAPHY 



while living micro-organisms are only shown up in suffi- 

 cient contrast to the field when the aperture is closed 

 to about J or -J ; diffraction is necessary to give them 

 distinctness, but it is very liable to be overdone in the 

 photomicrograph. Oil-immersion lenses can be used with 

 a smaller restriction of aperture than dry lenses, and 

 apochromats than achromats. 



11. Depth of Focus and Aperture. Examine a fairly 

 thick object (e.g. a large diatom), and note that as the 

 iris is closed a greater number of planes come into focus 

 at once, until diffraction interferes with the result. To 

 get as great a depth of focus as possible it is better to 

 replace the objective by one of lower N.A. rather than 

 restrict the aperture of the higher, or to use a lower power 

 objective and longer extension to get the same magni- 

 fication. 



1 2. Critical Illumination with Various Radiants. Obtain 

 critical light with whatever radiants are available, using 

 blackened glass to protect the eye from the glare of any 

 that are very intense, and examine the field of view and 

 the Ramsden circle in each case when using a moderate 

 objective (e.g. "). It will be found that only a few radi- 

 ants evenly fill the back lens of the objective. The edge 

 of the oil-lamp flame, a Nernst filament in some cases, 

 and a mercury vapour lamp, are satisfactory. Structure 

 can usually be made out in the image of an arc or lime- 

 light, and incandescent mantles always give a network 

 over the field. An ordinary electric lamp is worse than 

 any, showing its filament across the object, and it does not 

 fill the back lens. 



13. Introduce a piece of Ground Glass between the 

 Radiant and Microscope. A diffused light is obtained, 

 and the field becomes evenly illuminated. If the surface 

 of the ground glass is focussed as the source of light the 

 image is covered with a granulation. Ground glass is 

 never used when critical illumination is required. 



