OF THE MICROSCOPE. 137 



plate of metal, which may be raised by a screw, but when the 

 screw is withdrawn the "spring" of the plate carries it down 

 and gives the pressure. 



A compressorium, in which the pressure is caused by the 

 weight of the upper plate, is shown in Fig. 36. There are two 

 plates of metal, each with a hole in the centre, to receive the glass 

 discs. At one end the upper plate has two pins, which fit into 

 two holes in the lower plate, and serve to prevent all side 

 movements. A screw passes through the other end of the 

 upper plate, and serves to separate the two. A drop of water 

 containing an animalcule having been placed on the thin glass 

 attached to the lower plate, the upper plate, with the screw 

 projecting sufficiently from the under side, is laid on it. Then 

 by turning the screw, we can bring the two plates together to 

 any required degree of nearness, and with the utmost delicacy. 



Fig. 36. THE GRAVITY COMPRESSORIUM. 



Any minute animal may thus be firmly grasped, without crush- 

 ing it, while the compressing power exerted by the mere weight 

 of the metal plate is in almost all cases sufficient, even for the 

 complete flattening out of small worms, etc. Even such crea- 

 tures as the larva of the common gnat or mosquito may be 

 completely crushed by the weight of a plate less than the eighth 

 of an inch thick; and, where greater force is required, it is of 

 course easy to apply the pressure of the finger. In the latter 

 case, no danger of exerting too great a pressure need be in- 

 curred, as the projecting screw prevents all that. The want of 

 parallelism between the plates does not prove a serious objec- 

 tion, as it is so very slight that it is hardly perceptible in the 

 short distances ordinarily under observation. Where, how- 

 ever, it is desirable to avoid this defect, screws may be substi- 

 tuted for the pins, and the points may be made to work in 

 holes bored half through the lower plate. 



