232 PRACTICAL MICROSCOPY. 



micrometer is employed and a sketch made of it upon the 

 drawing slab at a distance of 10 inches. If, upon measuring 

 these lines with an ordinary rule divided into inches and 

 tenths, the hundredth of an inch space corresponds to 

 one linear inch, it is clear that the object has been magnified 

 100 diameters ; while if the thousandth of an inch space 

 occupied half an inch on the rule, it would have been 

 amplified 500 diameters. 



Eye-piece micrometers are of several kinds, the simplest 

 being a circle of glass ruled with a series of squares ^V to 

 TFJJ- of an inch (or T V of a millimetre) apart, being made 

 to rest on the diaphragm of the eye-piece, or placed in 

 front of the field lens in Ramsden's pattern. Previous to 

 using this form of micrometer the value of the sides of 

 these squares, with each objective, will have to be deter- 

 mined ; this is done in the following manner : Place a 

 stage- micrometer, ruled to hundredths and thousandths of 

 an inch (or tenths and hundredths of a millimetre), in posi- 

 tion upon the stage, and view it with the eye-piece having 

 the ruled circle in situ. The lines upon both micrometers 

 must now be rendered parallel, and carefully observed 

 where the one overlaps the other, and if any uneven 

 divisions appear, it will be better to pull out the draw- 

 tube until they become even, keeping a record as to how 

 much the tube has been withdrawn. As an instance 

 when using the i-inch objective and an A eye-piece, 

 each -j-Fff of an inch of the stage micrometer nearly 

 coincided with ten divisions of the ruled glass circle. In 

 order to make the coincidence exact, the draw-tube was 

 pulled out -nr of an inch ; therefore, for that objective and 

 ocular, each division of the eye-piece micrometer represents 

 -r^j- of an inch, when the draw-tube is extended the 

 same amount. 



Jackson's eye-piece micrometer is shown in Figs. 202 



