Petrographic Microscope Work. 



69 



but there are also rays which reach the section from other 

 directions. The fact that rays from one direction preponder- 

 ate gives rise to the characteristic phenomena produced by 

 obliquely incident light while the remaining rays only tend 

 to veil these phenomena and to render them less distinct. Jn 

 this respect this method is inferior to the first three methods. 



(5) When the condenser is focnssed on the object plane of 

 the draw-tube system, the lower focal plane of the condenser is 

 conjugate to the upper focal plane of the objective (tig. T) ; a 

 stop placed in the upper focal plane of the objective functions, 

 therefore, similarly to a stop in the lower focal plane of the 

 condenser,* and if properly placed should produce equally 

 satisfactory oblique illumination. In actual practice, however, 

 this method is not satisfactory because of the difficulty of 



Fig. 11. 



inserting the stop in the upper focal plane of the objective. 

 This focal plane, moreover, is not a plane but a warped surface 

 and covers only a small area. In many objectives the upper 

 focal plane is located within the lens system and is accordingly 

 out of reach entirely. In low power objectives of small numer- 

 ical aperture, moreover, the angle of inclination of the most 

 obliquely incident rays attainable is too small to be of much 

 service. In view of these objections the method can hardly 

 be recommended for general use. 



(6) In fig. 7 the image A 11 !) 11 is conjugate to AB, and a stop 

 placed at A"B IT should function similarly to AB itself. Theo- 

 retically this statement may be true, but practically it does not 



*H. Ambronn (Ber. Sachs. Gesell. d. Wissen. Math. phys. Kl., 1896, 1-6) 

 has developed a method for measuring relative refractive indices, which is 

 based on the phenomena produced in the upper focal plane of the objective. 



