OF MICROSCOPIC OBJECTS. 39 



together firmly at each end. It is evident that any object 

 placed betwixt them will be subject to continual pressure. 

 The power of this may be regulated by the thickness and 

 length of the whalebone. This simple contrivance is very 

 useful. 



Almost every scientific man, however, has his own model, 

 and it may be as well to examine one or two of them. Mr. 

 Goode uses the following : A, a piece of wood 8 in. long 

 and f in. thick. B, a spring made with thin iron wire. 

 The end of the spring is driven into the table, as at C. A 

 piece of ^-in. iron wire is then run through the springs, which 

 forms an axis to work upon, and also keeps them in theii 

 places. He inserts a pin at the side of the spring, so that 

 it will fall on a given spot, and not rub the cover from side 

 to side. The springs are made by binding the thin wire 

 round the J-in. rod about four or five times. 



