86 PHOTOMICROGRAPHY 



varies correspondingly from 300 or 400 to several 

 thousand candle-power. The most useful for the purpose 

 under consideration are the smaller types taking 5 

 amperes or less ; occasionally the larger lamps may be 

 employed with advantage, for cinematographic ex- 

 posures for example, but as a rule no greater illumination 

 than 500 candle-power is required. The small lamps 

 have the very great advantage that they can be run 

 with safety on ordinary wiring fitted with 6-ampere 

 fuses. Most supply-circuits are run at voltages of 110 

 to. 250, and as arc lamps only require 20 to 65 volts, 

 according to their power, a resistance must be connected 

 to them to reduce the pressure to that extent. Suitable 

 Resistances or Choking Coils are supplied by the lamp- 

 maker according to the voltage of the supply and its 

 periodicity if alternating ; they, of course, involve a 

 considerable waste of current by conversion into heat. 



When obliged to work on an alternating supply a 

 rectifier should be installed; of these the Cooper-Hewitt 

 mercury rectifier is very suitable, and is supplied for 

 any voltage and periodicity ; a Nodon rectifying valve 

 or a motor transformer can also be employed. Any 

 of these devices involves considerable outlay and it is 

 frequently advisable on that account to turn to some 

 other source of illumination for purposes for which the 

 alternating arc is unsatisfactory. 



The lamp must be so constructed that the crater 

 which forms on the positive carbon, and is the source of 

 light, faces the microscope with as little obstruction as 

 possible from the negative carbon, and in such a way as 

 to avoid loss of light backwards. In some lamps the 

 carbons are at right angles, the positive being horizontal 

 and the negative vertical ; in others there is a smaller 

 angle between them, while in others they are almost in 

 line, but inclined at about 30 to the vertical with the 

 lower negative carbon slightly in front of the upper 



