4 8 L[GHT[NG AND FOCUSING. \_CH. II 



form of illumination is only successful with low powers and objectives 

 of small aperture. It is well to make the illuminator immersion for 

 this experiment, see § 98. 



(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 graphite, use a 16 mm. (fi in.) objective, 

 and when the light is sufficiently oblique the lines will appear some- 

 thing 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 would be focused on the object (see § 82) 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 r /c 

 alcohol on the middle of a slide and allow it to dry and 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 dia- 

 phragm eccentric. The same specimen may also be tried with a mir- 

 ror 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 adding a 

 very small drop of the solution to the dried preparation, putting it under 

 the microscope very quickly, lighting for dark-ground illumination and 

 then watching the crystallization. 



ARTIFICIAL ILLUMINATION. 



§ 89. For evening work and for regions where daylight is not suf- 

 ficiently brilliant, artificial illumination must be employed. Further- 

 more, for the most critical investigation of bodies with fine markings 

 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 one made 

 of metal with a slit-like opening covered with an oblong cover-glass is 

 more satisfactory, as the source of light is more restricted. Very ex- 

 cellent results may be obtained, however, with the ordinary bed-room 

 lamp furnished with the usual glass chimney. 



The new acetylene light promises to be the most perfect of all the 

 artificial lights for microscopic observation and for photo-micrography. 

 (See under Photo-micrography). 



