230 PRACTICAL MICROSCOPY. 



smaller infusoria, do not give sufficiently positive outlines 

 for this method of display. Neither oil lamps nor the 

 paraffin lamp of the Sciopticon or Triplexicon yield suffi- 

 cient light. For magnifications of from 50 to 100 diameters 

 lime-light will perform well, but for higher powers the 

 electric light should be used. 



MiCROMETRY. The micrometer, as its name signifies, is 

 used for measuring small objects, and omitting all historical 

 notices which may, however, be found in the works of 

 Baker and Quekett we will proceed to describe those 

 forms most in use. 



FIG. 201. 



The stage micrometer consists of a glass slip such as is 

 shown in Fig. 201, upon which are ruled a series of lines 

 generally -j^j- of an inch apart, one of these divisions 

 being again divided into ten parts ; so that there is on the 

 same scale the means whereby objects of varying magni- 

 tude may be exactly measured, to less even than the 

 thousandth of an inch. 



Some micrometers are also ruled to millimetres, tenths 

 and hundredths, and this system is daily coming more into 

 use. A standard system for measurements was long 

 required for scientific microscopical purposes, and by very 

 general consent the thousandth part of a millimetre has 

 been taken as this standard, and by some termed a micro- 

 millimetre ; while others, objecting to the prefix " micro," 

 which in electrical measurements means a millionth of the 

 unit, prefer to call the unit of the thousandth of a milli- 

 metre a micra. One thousand micras make one millimetre. 

 The following table, extracted from the 'Journal of the 



