CHAPTER IV. 



ILLUMINATION. 



ILLUMINATION ADVANTAGES OF ARTIFICIAL LIGHT 

 A SIMPLE OBJECT WITH A Low POWER' 

 DIATOMS WORKING WITH POLARISED LIGHT 

 OPAQUE OBJECTS DARK GROUND EFFECTS. 



Artificial light being more constant than daylight, 

 it follows that exact exposures may be repeated 

 without risk of failure. The landscape photo- 

 grapher has no power to alter the prevailing light 

 and shade, whilst the photo-micrographer can 

 modify them at pleasure. But, on the other hand, the 

 latter will find difficulty in getting any contrast at 

 all when dealing with very transparent objects, and 

 it is here that his individuality will be manifested. 

 The iris diaphragm will sometimes be needed ; at 

 other times it may be the polariscope, especially for 

 crystals, or perhaps the colour screen must be 

 employed. Which method and which colour screen 

 will only rightly be selected after experience has 

 been gained. If the worker prepares his own ob- 

 ject he can often stain it to bring out its details. 

 However, all these matters will have to be dealt 

 with separately. 



