Chap. X.] 



GALVANOMETER SCALE. 



109 



Fig. 57. Telescope and Scale. 



the boussole is placed a table, on which stands an 

 astronomical telescope. Above the telescope, sup- 

 ported on uprights, is a metre scale ss, which is divided 

 into centimetres and millimetres. Each centimetre 

 is marked with REVERSED numbers (Fig. 57). The 

 table is so placed 

 that the scale is 

 directly opposite 

 the mirror, and at 

 right angles to the 

 axis of the tele- 

 scope. With a 

 little trouble, the 

 position of table 

 and scale is so ar- 

 ranged that, on 

 looking through 

 the telescope, the 

 mirror of the boussole is seen, and the image of 

 the scale reflected in it, the numbers of the scale 

 being seen, of course, in the ordinary position. By 

 adjusting the scale with a rack and pinion its mark 

 can be brought into the centre of the field, and made 

 to coincide with the vertical thread of the telescope. 

 The distant pulley (p 2 , Fig. 56) of the accessory magnet 

 should be clamped to the telescope table. The slightest 

 movement of it will cause a deflection of the needle, 

 and this will be observed through the telescope, when 

 it will appear as if the scale were drawn across the 

 mirror. When the needle comes to rest, the reading, 

 through the telescope, of the number now reflected in 

 the mirror will indicate the amount of deflection. 



On the same table on which are placed the tele- 

 scope and stand may be fixed keys and other arrange- 

 ments, these being connected with the galvanometer 

 by long wires, carried out of the way, overhead, to the 

 instrument. 



