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Proceedings of the Royal Irish Academy. 



closes the circuit tlirough a galvanometer, not seen in the figure, but 

 situated inside the eyepiece of the microscope. The instrument I 

 used had not the microscope attached to the bed plate, but separate 

 from it ; otherwise the instrument was identically the same as that in 

 the figure. The forceps are bent over at the ends, allowing a trough 

 to be raised, and surround the ribbon when in position. This trough 

 can be lowered when necessary, and has been found very advantageous. 



as it helps greatly m excluding draughts, which are fatal to accurate 

 working. This trough, however, forms a dust-trap, and will give rise 

 to trouble if not very carefully and frequently cleaned, as the particles 

 of the minerals dealt with fall into it, and the slightest breath of air 

 blows them on the ribbon, wliich is thereby rendered too dirty, and so 

 useless for further determination. The rheostat used was the same as 

 that employed by Dr. Joly, except that German silver wire was used 



