54 ARTIFICIAL LIGHT FOR THE MICROSCOPE [Ch. II 



a 4.x or 5.x 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 (§ 47) is illuminated 

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

 ing into the microscope. It sometimes requires considerable manipu- 

 lation 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 concave mirror condenses the light, the field will look brighter 

 with it than with the plane mirror. It is especially desirable to re- 

 member that the excellence of lighting depends in part on the position 

 of the diaphragm (§ 88). 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 deter- 

 mined by finding the focal point of the mirror in full sunlight. 



§ 95. Use of the plane and of the concave mirror. — The mirror 

 should be freely movable, and have a plane and a concave face (fig. 

 20). 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 neces- 

 sary to have parallel rays or to know the direction of the rays. 



Experiments with Artificial Light and a Mirror 

 § 96. Lighting with a kerosene lamp. — For this a lamp with a 

 flat wick from 3 to 5 cm. wide is best. It should be turned up well, 

 but not enough to smoke. The face of the flame should be turned 

 toward the microscope for low powers. For moderate powers the 

 flame should be made oblique and for high powers the edge of the flame 

 should be used. This is because the thicker source of light gives a 

 greater brilliancy. Use the fly's wing or any well-stained preparation. 

 As the light is in diverging beams it is best to use the concave mirror 

 to partly overcome the divergence. One must learn by experience 

 and trial how far off to have the lamp. A distance of 15 to 20 cm. 

 is usually satisfactory. There should be an opaque screen between 

 the lamp and the microscope to protect the eyes of the observer and 

 to screen the stage of the microscope (fig. 33). 



