42 LIGHTING AND FOCUSING [ CII. II 



DIAPHRAGMS 



I 76. Diaphragms and their Proper Employment. — Diaphragms are 

 opaque disks with openings of various sizes, which are placed between the 

 source of light or mirror and the object. In some cases an iris diaphragm is 

 used, and then the same one is capable of giving a large range of openings. 

 The object of a diaphragm in general, is to cut off all adventitious light and 

 thus enable one to light the object in such a way that the light finally reach- 

 ing the microscope shall all come from the object or its immediate vicinity. 

 The diaphragms of a condenser serve to vary its aperture to the needs of each 

 object and each objective. 



ji 77. Size and Position of Diaphragm Opening. — When no condenser is 

 used the size of the opening in the diaphragm should be about that of the 

 front lens of the objective. For some objects and some objectives this rule 

 may be quite widely departed from ; one must learn by trial. 



When lighting with a mirror the diaphragm should be as close as possible 

 to the object in order, (a) that it may exclude all adventitious light from the 

 object ; (b) that it may not interfere with the most efficient illumination from 

 the mirror by cutting off a part of the illuminating pencil. If the diaphragm 

 is a considerable distance below the object, (1) it allows considerable adventi- 

 tious light to reach the object and thus injures the distinctness of the micro- 

 scope image; (2) it orevents the use of very oblique light unless it swings 

 with the mirror ; (3) it cuts off a part of the illuminating cone from a concave 

 mirror. On the other hand, even with a small diaphragm, the whole field 

 will be lighted. 



With an illuminator or condenser (Figs. 47, 54), the diaphragm serves to 

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

 the aperture (see §95). 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 pro- 

 produced (f! 103; Fig. 57). 



ARTIFICIAL ILLUMINATION 



J 78. For evening work and for certain special purposes, artificial illumi- 

 nation 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 (seeCh. 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 EXPERIMENTS 



§ 79. Lighting with a Mirror. — As the following experi- 



