188 THE MICROSCOPE. 



those of Mr. Okeden, Mr. Tyrrell, Mr. Amyot, &c., 

 being used. The first named (Mr. Okeden's) finder 

 consists of two graduated scales, one of them vertical, 

 attached to the fixed stage-plate, and the other horizontal, 

 attached to an arm carried by the intermediate plate ; the 

 first of these scales enables the observer to " set " the 

 vertically-sliding plate to any determinate position in 



Fig. lll.Amyot's Object Finder. 



relation to the fixed plate, while the second gives him the 

 like power of setting the horizontally-sliding plate by the 

 intermediate. The finder the author has been in the 

 habit of using for some years is a very ingenious and 

 effective instrument described by Mr. Amyot. It consists 

 of a box-wood plate, with a circular hole ^-ths of an inch 

 in diameter cut out of its centre, into which a disc of 

 ivory, pierced in the exact centre with a very fine needle, 

 fits ; this disc can be readily removed by seizing a little 

 brass peg, attached to it, with the forceps. The box-wood 

 is ruled with two axes, vertical and horizontal, which are 

 graduated to 50ths or lOOths of an inch. The diagram 

 (fig. Ill) represents the finder with bone centre-piece in 

 situ. The dotted ring shows the rabbet on which the 

 centre-piece rests ; a, brass pin attached to centre-piece ; 

 b, b, two brass pins for steadying the instrument against the 



