164 MICROSCOPIC OBJECTS. 



suit the thickness of any object. When required for high 

 power the cover should have a very thin plate of glass to 

 permit the magnifier to approach closer to the object. In 

 the larger boxes a small hole is made in the side of the 

 cover for the escape of air and superfluous water, which 

 is afterwards closed with bee's wax. 



To render these aquatic boxes more useful the bottom 

 glass should have a series of lines cut on its superior sur- 

 face for measuring the size of the object, which in this 

 manner is done without any additional trouble, and ren- 

 ders them a valuable addition to a microscope, serving 

 the purpose both of a micrometer and object-holder. 

 The divisions most useful are the one hundred in an inch. 

 In using these micrometers the observer wiU do well to 

 rub into the hues a little black lead, by taking a pencil 

 and rubbing the point obliquely across them. 



Observers often find it a difficulty to ascertain which 

 side of the glass has the lines engraved on it : but 

 although the eye cannot detect them easily, they may 

 always be ascertained readily by rubbing the finger-nail 

 across them. 



The most complete live-boxes for animalcules are 

 those having a small disc of glass cemented to the lower 

 glass. This disc may be divided into micrometer squares 

 if desirable. The chief use of the disc is to prevent the 

 di'op of water from spreading : thus infusorial animalcules 

 cannot swim beyond the edge of the disc, and hence they 

 can always be brought mthin the field of view of the 

 microscope. 



