PHYSICS: A. G. WEBSTER 
175 
vertical image of the filament is drawn out into a horizontal band. Details 
are shown in Fig. 2. The aluminium disk 12 is hung in the center of the hole 
in the front plate by means of the three steel wires clamped at one end by the 
spider 13, and at the other wound on the pins 8, turned by a key from the 
FIG. 1 A 
PHONOMETER 
AG WEBSTER 
FIGS. 1 and 2 
outside, one controlled micrometrically by a lever and screw 17, by which the 
fine tuning is done. The wires pass over either one of two sets of three bridges 
cast on the front plate. The objective of the telescopic observing device is 
