76b DARK-GROUND ILLUMINATION WITH HIGH POWERS [Ce. II 



step-down transformer must be used. There is practically no loss 

 in the transformer so that this is an economical method. If direct 

 current only is available then it can be used to charge a storage bat- 

 tery, and the current for the 6-volt lamp drawn from the storage 

 battery yielding a six-volt current. A step-down transformer cannot 

 be used with direct current, only with alternating. 



FIG. sod. HEADLIGHT LAMP- HOUSE, WIRING AND STEP-DOWN TRANS- 

 FORMER. 



(About one eighth natural size.) 



D, Glass Window of daylight glass on the side of the lamp house to be used 

 for bright- field microscopy. 



P. lens Parallelizing lens of about 75-mm. focus. It is cemented to a piece of 

 daylight glass. 



me Mistakeless connection between the lamp wires and those from the sec- 

 ondary of the transformer. It is different from any connection for a no- volt 

 circuit. 



6 Volts. The secondary side of the transformer where the voltage has been 

 stepped down from no to 6. 



no Volts. The primary side of the transformer where the voltage of the cur- 

 rent in the supply wires is no. 



Transf. The transformer, stepping the voltage down from no to 6. 



The filament of the 6-volt lamp is very compactly coiled so that it 

 is about the size of the crater in an arc lamp. The lamp found 

 sufficient for all purposes requires 108 watts. One of 72 watts an- 

 swers for most purposes, but is not so satisfactory as the io8-watt 

 one (fig. 5oc). As shown in the illustrations, the headlight lamps 

 should be enclosed in a lamp-house, and have an adjustable fixture 

 so that they can be centered with the parallelizing lens and also focused 



