64 



F. E. Wright — Oblique Illumination in 



These different methods will now be considered briefly and 

 with special reference to their suitability for use in practical 

 work. 



(1) In this method, which has frequently been used,* the 

 condenser and polarizer are first removed from the microscope 

 and the mirror swung to one side from which the reflected 

 beams of light enter the section under an incident angle of 

 50° under favorable conditions (fig. 1). If the polarizer be 

 allowed to remain on the microscope, this angle of incidence 

 is materially decreased (to less than 15°) unless the nicol car- 



riage be specially constructed so that it can be swung together 

 with the mirror, f 



For practical work this method for obtaining oblique illumi- 

 nation is hardly to be recommended in view of the bother con- 

 nected with its application. It does, however, furnish a fairly 

 parallel beam of light under a relatively large angle provided 

 the nicol and condenser are not present on the microscope. 

 The intensity of illumination obtained under these conditions 

 is not great. 



* Schroeder van der Kolk, Zeitschr. Wiss. Mikroskopie, viii, 456-458, 1892. 

 t Schroeder van der Kolk, ibid., viii, 458, 1892. 



