53 



LIGHTING AND FOCUSING 



[CU. II 



very oblique light may be used, employ a stage micrometer in 

 which the lines have been filled with graphite, use a 16 mm. 

 ( 2 3 in.) objective, and when the light is sufficiently oblique the 

 lines will appear something like streaks of silver on a black back- 

 ground. A specimen like that described below in (B) may also 

 be used. 



(B) With the Abbe Condenser. — Have the illuminator so that 

 the light is focused on the object (see § 97) and use a diaphragm 



Fig. 58. Abbe Condenser of r. 20 Fig. 59. Abbe Condenser of 1.40 



N.A. in section. N.A. in section. 



Cuts loaned by Voigtlander &Sohn, A.G. 



with the annular opening (Fig. 57); employ the same objective as 

 in (A). For object place a drop of 10 % solution of salicylic acid 

 in 95 "0 alcohol on the middle of a slide ; it will crystallize. The 

 crystals will appear brilliantly lighted on a dark back-ground. Put 

 in an ordinary diaphragm and make the light oblique by making 

 the diaphragm eccentric. The same specimen may also be tried 

 with a mirror and oblique light. In order to appreciate the differ- 

 ence between this dark-ground and ordinary transmitted-light illu- 

 mination, use a central diaphragm and observe the crystals. 



A striking and instructive experiment may be made by 

 adding a very small drop of the solution to the dried preparation, 

 putting it under the microscope quickly, lighting for dark-ground 

 illumination and then watching the crystallization. 



§ 103a. Dark-Ground Illumination for High Powers. — 

 There are two methods for making objects appear as if self lumin- 

 ous in a black field : (1) To light the objects by rays so oblique 

 that none of them will enter the objective unless they are deflected 

 by some object in the field. This method was employed above for 

 low powers. For high powers very wide apertures must be used 

 for the condenser. No rays below 1.00 N. A. can be successfully 



