64 ELEMENTARY PHOTO-MICROGRAPHY. 



of error only detected by an experienced micro- 

 scopist. 



The writer has never found the Lieberkuhn of 

 any advantage in illuminating opaque objects. A 

 reflector called a vertical illuminator may be tried 

 when the lens approaches so near to the object that 

 it is difficult to project a beam of light from the 

 side reflector. It is especially useful for photo- 

 graphing metals with high powers or objects 

 mounted dry on cover. 



MULTIPLE COLOUR ILLUMINATION. 

 A method of multiple colour illumination has 

 been devised by Mr. Rheinberg, by which it is 

 possible to cause an object and its background to 

 appear of different colours in a striking manner, 

 and so secure a greater contrast than usual. Illus- 

 trations and particulars of this method are given 

 in the " Illustrated Annual of Microscopy" for 1898. 

 Briefly stated, it consists of very thin sheet gelatine 

 of various colours cut into annular rings and 

 central discs to fit the stop holder of condenser 

 exactly as when dark-ground illumination is 

 employed, and such as any amateur could easily 

 cut out for himself. In other words, the circular 

 disc has a hole punched through its centre, which 

 is then filled up by a piece of a different colour as 

 if a threepenny-piece were placed on a halfpenny, 

 the copper being, of course, removed from under 



