. The microscope : an introduction to microscopic methods and to histology. Microscopes. 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. (23 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
. The microscope : an introduction to microscopic methods and to histology. Microscopes. 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. (23 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 Fig. 59. Abbe Condenser of in section. in section. Cuts loaned by Voigtlander &Sohn, 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 N. A. can be successfully. Please
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