. The microscope; an introduction to microscopic methods and to histology. Microscopes. 5° LIGHTING AND FOCUSING [CH. II. oblique rays will pass into the objective, hence as light reaches the objective only from the object, all the surrounding field will be dark and the object will appear like a self-luminous one on a dark back- Fig. 52. An Abbe Condenser in its mounting ( The Bauseh & Lomb Optica/ Company). ground. This form of illumination is most successful with low powers. It is well to make the illuminator immersion for this experiment, (see § 105). (A) With the Mirror—Remove all the
. The microscope; an introduction to microscopic methods and to histology. Microscopes. 5° LIGHTING AND FOCUSING [CH. II. oblique rays will pass into the objective, hence as light reaches the objective only from the object, all the surrounding field will be dark and the object will appear like a self-luminous one on a dark back- Fig. 52. An Abbe Condenser in its mounting ( The Bauseh & Lomb Optica/ Company). ground. This form of illumination is most successful with low powers. It is well to make the illuminator immersion for this experiment, (see § 105). (A) With the Mirror—Remove all the diaphragms so that very oblique light may be used, employ a stage micrometer in which the lines have been filled with graph- ite, 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 § 86) and use a diaphragm with the annular opening (Fig. 51); employ the same objective as in (A). For object place a drop of 10 % solution of salicylic acid in 95 % 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 difference between this dark- ground and ordinary transmitted-light illumination, use an ordinan' diaphragm and observe the crystals. A very striking and instructive experiment may be made by add- ing 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. ARTIFICIAL ILLUMINATION § 93. For evening work and for regions where daylight is not suffic
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