36 LIGHTING AND FOCUSING. [CH. II. 



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

 ■or for any special purpose to be served (see \ 80). 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 

 (?88; Fig. SO- 

 ARTIFICIAL ILLUMINATION. 



§65. For evening work and for certain special purposes, artificial illumination 

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

 very satisfactory, but for brilliancy and for the actinic power necessary for photo- 

 micrography (see Ch. VIII) the new acetylene light seems to be all that could be 

 desired. Whatever source of artificial light is employed, the light should be bril- 

 liant and steady. 



LIGHTING AND FOCUSING : EXPERIMENTS. 



§ 66. Lighting with a Mirror. — Place a mounted fly's wing under 

 the microscope, put the 16 mm. { 2 /i in.) or other low objective in posi- 

 tion, 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 (§ 46) 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 

 (§ 57)- If tne 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. 



§ 67. Use of the Plane and of the Concave Mirror. — The mirror 

 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. 



§68. Focusing with Low Objectives. — Place a mounted fly's 

 wing under the microscope ; put the 16 mm. (fj in.) objective in posi- 

 tion, and also the lowest ocular. Select the proper opening in the dia- 

 phragm and light the object well with transmitted light (§ 6o, 64). 



