THE OPTICAL EQUIPMENT 



69 



In photo -micrography too much attention cannot be given to 

 illumination, for on this more than on any other factor depends 

 the perfection of the result. No apology is therefore offered 

 for having given a rather lengthy and detailed description 

 of practically all illuminating appliances available at the 

 present time. In succeeding portions of this book where 

 the method of photographing special objects is described, the 

 method of illumination will in all cases be as fully set out as 

 is practicable. 



FIG. 19. 



-Vertical Illuminator for Opaque Objects. 

 (E. Leitz of Wetzlar.) 



The Collecting-lens System. The satisfactory illumination 

 of microscopic objects for photographic purposes is not without 

 certain difficulties which are due to the size and shape of the 

 illuminant. The field of view must be perfectly evenly illu- 

 minated, and as a rule the illumination must be very much more 

 intense than is the case in visual work. Most illuminants of 

 high power are small in area ; consequently where a critical 

 image is desired they do not sufficiently cover the field, and 

 the illumination is therefore not equal. For this reason some 

 auxiliary collecting lenses are necessary, and a description of 

 the principal types of these follows. 



The simplest form of collecting-lens to intervene between 

 the source of light and the sub-stage condenser of the microscope 

 is what is known as the bull's-eye. This is a plano-convex 



