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. ( 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 § 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 

 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, 



