68 



F. E. Wright — Oblique Illumination in 



is viewed simultaneously and in sharp focus with the object. 

 No blurred edges are visible. If now the condenser system be 

 lowered (fig. 9) or raised (fig. 10), the finger edge becomes 

 indistinct and blurred. The paths of the rays are indicated in 

 the figures and require no further explanation. 



If now a mineral grain (indicated as of irregular outline in 

 the object plane of the figures) be observed with the condenser 

 and finger in the position of fig. 8, it appears illuminated by 

 light from all directions and no effects of oblique illumination 

 are visible. In fig. 9, however, the rays which impinge on it 



Fig. 8. 



Fig. 9. 



Fig. 10. 



are included in the indistinct edge of the shadow and these 

 travel from left to right in the figure, while in fig. 9 their 

 direction is from the opposite side, right to left ; but in both 

 cases oblique illumination is obtained, though not over so large 

 an area as in the second method. 



(4) This method was first used by Schroeder van der Kolk,* 

 who placed a metal strip over the upper lens of the condenser 

 as indicated in fig. 11. From this figure it is evident that the 

 oblique illumination obtained by this method is less perfect 

 than that of the foregoing methods. There is a greater num- 

 ber of rays from one particular direction which are not cut off, 



*Zeitschr. Wiss. Mikroskopie, viii, 456-458, 1892; Kurze Anleitung zur 

 mikroskopischen Krystallbestimmung, Wiesbaden, 1898. 



