164 A. TANAKADATE AND H. NAGAOKA. 



natural size. 



^ natural size. 



Natural Size. 



the deflecting magnet and to restore it always to the same place, four 

 stops a, h, a, h' are rigidly fixed to the rod. These stops are triangular 

 prisms of brass fitting into the V. Each has a slightly oblique section. 

 It is so fixed to the rod that tlie magnet comes in contact with the 

 acute edge of the prism along the diagonal of the square section. 

 l>efore placing the magnet on the V, the groove was always carefully 

 swept with a camel's hair brush, so as to make sure of contact, both 

 along the groove, and against the stop. The magnet was placed in 

 such a position that the lateral fice with the spot before spoken of 

 was always turned from the observer. The magnet was slid by the 

 end of the brush along the groove. In transferring it from one to the 

 other side of the rod, or in reversing it, care was taken to guard 

 against change of temperature from handling, by wrapping it in a 

 piece of folded crape. 



The temperature of the rod was read by means of two thermo- 

 meters (Salleron No. 1386 and No. 15476), each graduated to | of a 

 degree. They were loosely attached, one to each branch of tlie rod. 

 One of the thermometers was placed in ft-ont of the observer, and tlie 



