90 PEACTICAL PHOTO-MICKOGKAPHY 



is switched on, owing to the necessity for this preliminary 

 heating, but this does not present any particular disadvantage 

 for microscopic work. The time required for the lamp to attain 

 its full lighting efficiency is only about thirty to forty seconds. 



With the one-ampere lamp, already mentioned, the filament 

 is sufficiently thick to form a source of light that will evenly 

 illuminate a great portion of the microscopic field, and be 

 sufficient at least for moderate or high powers. 



The lamp must of course be suited to the voltage of its 

 circuit, but the candle-powers for which the lamps are made 

 and sold vary almost directly with the voltage of the supply. 

 Hence a lamp for 100 volts will only give about half the candle- 

 power of a lamp for a 200-volt supply. 



There is a regulating resistance placed in series with the 

 filament, as it is rather liable to fracture with sudden changes 

 of electrical pressure, and consequently this resistance has a 

 steadying effect. The resistance absorbs about twenty volts, 

 the remainder of the electrical energy being utilised in the 

 production of light. 



The lamp is supplied in a very convenient form which can 

 be easily supported on any adjustable stand, such as a retort 

 stand, the filament being then in a vertical position. The 

 adjustable lime-light tray already described with vertical 

 and horizontal movements, may also be easily adapted, and is 

 the most satisfactory form of support. 



The type of lamp that should be purchased is that known 

 as the ' Luna ' Nernst lamp, and not those in which the fila- 

 ment is bent, or is surrounded by a spiral heater. Its initial 

 cost is low, and the cost of running is comparable with any 

 ordinary carbon filament lamp, its efficiency being very high. 



The candle-power measured in the direction of the axis of 

 the lamp is as follows : 



Voltage. 1 Amp. ' Luna ' Lamp. 



200 = 200 candle-power 



220 = 250 



240 = 274 



260 = 296 



It is essential in connecting up that the negative pole of 

 the source of supply should be connected to the negative 



