ACCKSSOEY INSTRUMENTS. 137 



a pair of blades may be applied to them for the purpose of 

 cutting off portions of these plants close down to the roots, 

 even in tall jars that are too small to admit of the intro- 

 duction of the hand. 



COMPKESSOEIUM. 



The compressorium is an instrument by which objects may 

 be gradually compressed between two parallel plates of glass. 

 The pressure may be applied whilst the object is being ex- 

 amined with the microscope, and may be kept up at will, so 

 that the alteration which it assumes, as the pressure is being 

 applied, can be observed with facility ; it is extremely useful 

 for crushing or compressing such objects as are so thick that 

 the light cannot readily be transmitted through them, or for 

 making flat others the elasticity of which is sufficient to raise up 

 the thin cover when they are placed between glasses to be viewed 

 in the ordinary way. There are many kinds in use, some of 

 foreign, others of home invention. The most simple, and the 

 one in which the power employed cannot exceed the force of 

 two spiral springs, is made by Mr. Smith, after a plan of Mr. 

 Lister's, and is represented by fig. 91. It consists of a bottom 



Fig. 91. 



plate of brass, to the centre of which is attached a piece of tube 

 having on its outside a short screw, on which works a large 

 circular nut, with a milled head ; to the inside of the tube a 

 circular piece of plate-glass is fixed, projecting slightly above 

 its edges ; this may be called the object-plate ; two small up- 

 right rods, fastened into the bottom plate, are provided with 

 spiral springs, their tops being surmounted by small nuts, 

 which keep the springs in place. A plate of brass, with a 

 hole in it larger than the object-plate, is made to shde up and 

 down the rods in a state of parallelism, by means of the large 



