48 ARTIFICIAL ILLUMINATION [Ch. II 



The object of a diaphragm is to cut off adventitious light and to 

 vary the aperture to suit the object and the objective. 



§ 89. Size and position of the diaphragm with a mirror only. — 

 When no condenser is used in addition to the mirror, a diaphragm 

 opening about the size of the front lens of the objective may be 

 employed. Its position may be close. to the object, in which case it 

 admits the greatest aperture of light, and cuts off the most adventi- 

 tious light; in this position it hghts the smallest field, however. 



If the diaphragm is far enough below the object the field may all 

 be lighted, but the aperture wiU be smaller than when it is close to 

 the object, as one may see by removing the ocular and looking down 

 the tube into the back lens of a i6 mm. or 8 mm. objective. On the 

 other hand, while the aperture of the objective may be filled even with 

 a small diaphragm opening close to the object, the field of view (§ 47, 

 fig. 65) may be but partly lighted. In that case the opening must 

 be increased until the entire field is illuminated. One must learn 

 by practice how to get the best effects. 



§ 90. Diaphragm with condenser. — The diaphragm with a con- 

 denser serves to vary the aperture of the cone of light to adapt it 

 to the objective, and to the object. 



If the opening in the diaphragm is not in the axis of the condenser 

 the object will be unsymmetrically illuminated; the object will also 

 be unsymmetrically illuminated if the diaphragm is wide open but 

 the Ught blocked from one side by placing an opaque body, like the 

 finger, between the mirror and the diaphragm (fig. 46, 67). 



The diaphragm is below the condenser in many forms, but between 

 the lenses in some (fig. 39-42). 



Artificial Illumination 

 § 91. Artificial light. — While daylight is to be preferred for most 

 microscopic as for other exacting work, it is not always possible to 

 work by daylight, and then sometimes one's work room or laboratory 

 is so situated that, even in the daytime, artificial light must be em- 

 ployed. For some purposes, like photo-micrography, it is desirable 

 to have a very uniform light, and this is gained most readily by using 

 some form of artificial light. 



