F. F. Wright — Oblique Illumination. 63 



Akt. VIII. — Oblique Illumination in Petrographic Micro- 

 scope Work ; by Fred. Eugene Wright. 



In ordinary microscope work three kinds of illumination are 

 in common use — central, oblique, and dark-ground. Of these 

 the last is employed only with high magnifications and then to 

 emphasize the outlines of minute particles. It is rarely applied 

 to petrographic microscope work because of the relatively low 

 magnifications there required. Oblique illumination is also 

 often disregarded by penologists, who employ central illumi- 

 nation either by parallel or convergent light to the practical 

 exclusion of all other types. There are many instances, how- 

 ever, where oblique illumination is better adapted to bring out 

 certain optical phenomena than either central or dark-ground 

 illumination and where it should accordingly be adopted. In 

 the following paragraphs it is proposed to indicate a few of 

 these uses and to explain briefly certain details of the methods 



Fig. 1. 



OBJECT PLANE C 



REFLECTOR 



which, in part at least, seem not to have been considered here- 

 tofore. 



Methods for Producing Oblique Illumination. 



(1) By swinging the substage mirror to one side and allow- 

 ing the light to enter the section under a large angle (condenser 

 and polarizer having been previously removed) ; 



(2) by a sliding stop in the lower focal plane of the con- 

 denser ; 



(3) by placing the index finger below the condenser and 

 observing the edge of the shadow which it casts ; 



(4) by means of an opaque strip immediately above the 

 upper lens of the condenser ; 



(5) by use of a sliding stop in the microscope draw tube ; 



(6) by a sliding stop in the eye circle of the ocular. 



