164 H. J. Denham 
writer 1 for normal microscope illumination, but works equally well 
for projection. 
It is quite unnecessary to buy a special resistance for a Pointolite 
outfit, as these can be made up more simply and cheaply in any well- 
equipped laboratory. The published figures for the 100 c.P. are as 
follows: 
Resistance in ohms 
Volts 
A 
B 
100 
12 
30-2 
no 
13-3 
36-7 
200 
2 5*5 
93-5 
220 
28 
106*5 
240 
3 i 
119 
and the only part that must be purchased from the makers is the 
special three-contact lampholder. 
The resistances for the 30 c.P. are not published, but can be con¬ 
structed from the data that the ioniser circuit requires 4-5 amps, (for 
five seconds when starting), and the arc circuit 0-45 amp. A resist¬ 
ance supplied by the makers for 200 volts has a resistance of 42 ohms 
1 Nature, Nov. 17, 1921, p. 369, “Microscope Illumination and Fatigue.” 
