Willson — Mode of Beading Mirror Galvanometers, etc. 51 



horizontal line which crosses the center of the movable mir- 

 ror ; usually the reflecting portion is placed above, as shown in 

 fig. 1, where a and o represent the silvered and nnsilvered por- 

 tions of the fixed mirror and c the movable mirror. 



The instrument is so adjusted that the movable mirror when 

 in its position of rest is nearly parallel to the fixed mirror. 

 The scale used, as shown in £g. 2, has its middle division line 

 prolonged downward to form an index ; this scale being brought 

 in front of the mirror, the eye is placed at the proper height 

 to see, as in fig. 3, the image of the scale in the upper fixed 

 mirror, the lower ends of the divisions coincident with the 

 lower edge of the silvering, while the index is seen nearly 

 continuous with the middle line of the scale, but reflected from 

 the movable mirror ; any deflection of the latter causes the 

 image of the index to move along the lower edge of the re- 

 flected scale by an amount corresponding to the double angle 

 of deflection.* 



It is obvious that this construction does not interfere with 

 the use of the instrument with lamp, or telescope and scale, 

 the lower portion of the fixed mirror acting as the usual cover- 

 ing glass ; it is then desirable, however, to give the latter a 

 slight forward inclination to avoid double reflection ; a suffi- 

 cient inclination may be given with the leveling screws, with- 

 out interfering with the mode of reading, above described, 

 unless the lines of the graduation are very short. 



* A simple geometrical consideration shows that if the scale reading be n, the 

 angle of deflection a. the distance of the scale from the fixed mirror A, and the 



distance between the two mirrors d, tan 2a = 



A— d> 



rected for the thickness of the glass in the usual way. 



A and d, if necessary, cor- 



