CH. II] LIGHTING AND FOCUSING 37 



With an illuminator or condenser (Figs. 41 , 48) , the diaphragm serves to narrow 

 the pencil to be transmitted through the condenser, and thus to limit the aperture 

 (see \ 84). Furthermore, by making the diaphragm opening eccentric, oblique 

 light may be used, or by using a diaphragm with a slit around the edge (central 

 stop diaphragm), the center remaining opaque, the object may be lighted with a 

 hollow cone of light, all of the rays having great obliquity. In this way the so- 

 called dark-ground illumination may be produced (§ 92 ; Fig. 51). 



ARTIFICIAL ILLUMINATION 



\ 69. For evening work and for certain special purposes, artificial illumina- 

 tion is employed. A good petroleum (kerosene) lamp with flat wick has been 

 found very satisfactory, also an incandescent electric or Welsbach light, but for 

 brilliancy and for the actinic power necessary for very rapid photo-micrography 

 (see Ch. VIII) the electric arc lamp or an acetylene lamp serves well. Whatever 

 source of artificial light is employed, the light should be brilliant and steady. 



LIGHTING AND FOCUSING : EXPERIMENTS 



§ 70. Lighting with a Mirror. — As the following experiments 

 are for mirror lighting only, remove the substage condenser if present 

 (see § 79, for condenser). Place a mounted fly's wing under the 

 microscope, put the 16 mm.(/3 in.) or other low objective in position, 

 also a low ocular. With the coarse adjustment lower the tube of the 

 microscope to within about 1 cm. of the object. Use an opening in 

 the diaphragm about as large as the front lens of the objective ; 

 then with the plane mirror try to reflect light up through the diaphragm 

 upon the object. One can tell when the field (§ 50) is illuminated, by 

 looking at the object on the stage, but more satisfactorily by looking 

 into the microscope. It sometimes requires considerable manipulation 

 to light the field well. After using the plane side of the mirror turn 

 the concave side into position and light the field with it. As the con- 

 cave mirror condenses the light, the field will look brighter with it than 

 with the plane mirror. It is especially desirable to remember that the 

 excellence of lighting depends in part on the position of the diaphragm 

 (§ 68). If the greatest illumination is to be obtained from the concave 

 mirror, its position must be such that its focus will be at the level of 

 the object. This distance can be very easily determined by finding the 

 focal point of the mirror in full sunlight. 



§ 71. Use of the Plane and of the Concave Mirror. — The mir- 

 ror should be freely movable, and have a plane and a concave face. The 

 concave face is used when a large amount of light is needed, the plane 

 face when a moderate amount is needed or when it is necessary to have 

 parallel rays or to know the direction of the rays. 



