610 



sit axis, which is rendered horizontal by the help of a riding 

 level. The aperture of the object glass is eight-tenths of an 

 inch; a glass scale, divided to the :f^oth of an inch, is fixed 

 in its focus ; and the eye tube is made to move across the 

 scale in a dovetail slide. 



The magnets are hollow cyHnders, each furnished as a 

 collimator with an achromatic lens, and a fine hne cut on 

 glass in its focus. There are four such magnets : two of 

 them being 3f inches long, and half an inch in exterior dia- 

 meter, and two 3 inches long, and three-eighths of an inch 

 in exterior diameter. The larger magnets are furnished 

 with a Y stirrup, in which they may be inverted ; the smaller 

 magnets have the ordinary tubular stirrup, with a suspension 

 pin and screw socket. A hollow brass cylinder, of the same 

 dimensions as the larger magnets, and carrying a small hol- 

 low cylindrical magnet within, serves to determine the 

 amount of torsion of the suspension thread ; it is likewise 

 fitted up as a collimator. 



There are two boxes, within which the magnets are to be 

 suspended. That belonging to the smaller magnets is a 

 rectangular box of copper, closed by mahogany sliding sides, 

 and having a circular aperture at each end filled with paral- 

 lel glass. It is 3^ inches long, l^ inches wide, and 1 inch 

 deep, internally ; and the thickness of the metal is a quarter 

 of an inch, so that it may act powerfully as a damper. A 

 suspension tube of glass, eight inches long, is screwed into 

 an aperture in the top of the box ; and is furnished with a 

 graduated torsion cap at top, and a sliding suspension pin. 

 This box is made to fit on the centre of the upper plate 

 of the circle, and is capable of removal at pleasure. The 

 box employed with the larger magnets is of woot^ , and of 

 the same form as the copper box, but somewhat larger. It 

 is detached from the instrument, but may rest on the same 

 stand, A small wooden piece with a mirror serves to illumi- 

 nate the magnet colHmator, either from above or from the 



