CH. II] DARK-GROUND ILLUMINATION WITH HIGH POWERS 760 



to give a parallel beam. This requires vertical and horizontal move- 

 ments. 



If one does not have available the 6-volt lamps and a transformer 

 or the proper storage batteries, it is possible to do much good work 

 with a stereopticon lamp of 100 watts 

 or if the large lamp-house is availa- 

 ble a 2 50- watt stereopticon lamp. 

 These can be used on the no- volt 

 circuit, alternating or direct, and can 

 be used in either the large lantern 

 with focusing and centering devices 

 and a parallelizing lens, or the 100- 

 watt stereopticon lamp can be used 

 in the Chalet lamp-house (fig. 5oe). 



Finally if one has a Chalet or 

 other lamp-house in which the or- 

 dinary, long-necked C-Mazda lamp 

 is used (fig. 37) one can get fairly 

 satisfactory results; but if much 

 work is to be done the io8-watt, 

 6-volt lamp in the centering and 

 focusing lamp-house is much more 

 satisfactory. As unmodified artificial 

 light is trying to the eyes of most 

 people, it is desirable to employ the 

 daylight glass filter in dark-field work. 

 As shown in fig. 5oc, this is accomp- 

 lished by cementing, with Canada 

 balsam, a clear, plano-convex lens 

 of about 6 centimeters in diameter 

 and 75-mm. focus to a piece of 

 polished daylight glass. This day- 

 light effect enables most workers to see minute details more clearly 

 than is possible with unmodified artificial light. 



See also the catalogues of the Bausch & Lomb Optical Co. and 

 of the Spencer Lens Co. for larpps to use in dark-field work. 



FIG. 506. CHALET LAMP ix 

 SECTION, WITH A IOO-WATT, MAZDA 

 STEREOPTICON BULB FOR DARK- 

 FIELD MICROSCOPY. 



(About one sixth natural size.) 



c Electric cable to the lamp- 

 socket (5). 



dg Daylight glass covering the 

 window at the right. 



ms loo-watt Mazda stereopti- 

 con lamp bulb. By means of the 

 separable cap (sc) and plug (sj>) 

 this short-necked bulb is lowered 

 so that its tungsten coil is at the 

 same level as the C-coil of the long- 

 necked bulb in fig. 37. 



sc Socket cap partly separated 

 to show the connecting prongs. 



sp. Separable attachment plug. 



v,v,v Ventilators. 



