180 Mr. G. Pierce on Indices of 



galvanometer, it is made a part of the suspended system in a 

 sort of radio-micrometer, mounted so as to be convenient for 

 exposure to electric radiation instead of to heat radiation. 

 The arrangement is shown in sectional diagrams in figures 

 2 and 3. 



Fig. 2 is a vertical section through the suspension and per- 

 pendicular to the field-magnet. Upon a base, B, provided 



Fig. 2. 



with levelling screws, is placed a parabolic, cylindrical reflector, 

 sawed from a block of wood and covered inside with a copper 

 reflecting surface. The cavity formed by this reflector is 

 marked R, and the opening of the parabola is at the right. 

 The element of the cylinder is vertical. Above the reflector, 

 and separated from it by a sheet of brass, is a horseshoe 

 magnet, M, with pole-pieces coming near together and pro- 

 jecting into the brass tube, T, which contains the fibre of the 

 suspension. Within the reflecting cavity the continuation of 

 this tube is of glass, shown by dotted lines. The parabolic 

 reflector has a focal length of 2 centim., and an orifice 16 

 centim. wide by 13 centim. high. The height of the pole- 

 pieces is 2*5 centim., and the distance from the bottom of the 



