50 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 Bausch & Lomb Optical 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 1 6 mm. (^ 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 ordinary 

 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 

 sufficiently brilliant, artificial illumination must be employed. Fur- 

 thermore, for the most critical investigation of bodies with fine mark- 

 ings like diatoms, artificial light has'been found superior to daylight. 



A petroleum (kerosene) lamp with flat wick gives a satisfactory 

 light. It is recommended that instead of the ordinary glass chimney, 



