THE RADIANT AND ILLUMINATION 4? 



the object we desire to photograph. The vast 

 majority of such specimens are transparent, 

 requiring the rays of light to be thrown 

 through them; this mode of illuminating 

 being termed transmitted light. Others, again, 

 will not permit the passage of light through 

 their substance requiring to be shown by its 

 reflection from their surfaces. This is known 

 as opaque illumination. Many transparent ob- 

 jects are shown best both as to structure and 

 beauty when brilliantly illuminated on a dark 

 background, which is effected by means of a 

 paraboloid or by printing from a positive to 

 be presently described. This is called dark 

 field illumination. Others, as many crystalline 

 subjects, sections of minerals, etc., require the 

 use of polarized light to properly display their 

 beauties and structural details. With others, 

 again, as most of the diatoms, the lighting 

 must be more or less oblique in order to fully 

 resolve their delicate markings. It will be 

 necessary to determine which of these classes 

 an object belongs to before commencing to 

 photograph it. Examples of the various 

 methods will be found in the succeeding 

 Chapter on Negative Making, wherein I 

 have striven to place practical processes be- 

 fore my readers in such simple but at the 



