Accessories. 65 



of the field, the outer edges appear more highly magnified 

 than the centre. 



3. The Eyepiece Micrometer and Stage Micrometer. The 

 stage micrometer, as previously described, is placed on the 

 stage, and a somewhat similar micrometer is put into the 

 eyepiece, generally divided into hundredths of an inch. On 

 focussing the stage micrometer the two sets of lines will 

 appear in the field at once. It is now desirable to ascer- 

 tain how many divisions of the eyepiece micrometer are 

 included between one of the spaces that is, T ^ of an 

 inch of the stage micrometer. Perhaps it will be found 

 that there will be several lines of the eyepiece and -a frac- 

 tion in that space, and in order that this fraction may be 

 obviated the drawtube should be slightly pulled out, which 

 will give, of course, an increased amplification, until a cer- 

 tain number of the lines on the eyepiece micrometer are 

 exactly equal to the space on the stage micrometer. We 

 will imagine that the number of eyepiece micrometer lines 

 that fill TT nr of an inch of the stage micrometer is five. 

 The stage micrometer is now removed, and the object to be 

 measured replaces it. The lines of the eyepiece micro- 

 meter will still be seen in the field, and bearing in mind 

 that five of these lines equal T tj-o of an inch, any part of 

 the object can at once be measured. It must be remem- 

 bered, however, that every objective requires an estimation 

 of the value of the eyepiece micrometer. 



To give greater facility and accuracy, a form of eyepiece 

 micrometer is used devised by Jackson, which is fitted in a 

 frame, and by means of a micrometer screw traverses the 

 object. If there be no mechanical stage to the instrument 

 it is very difficult to set a special part against the micro- 

 meter for measurement, especially with high powers. This 

 form of micrometer surmounts this difficulty. The ordinary 

 eyepiece micrometers can be used with ordinary eyepieces, 

 but the Jackson form requires that the outer tube of the 

 eyepiece shall be cut to receive the carrier for the micro- 

 meter. This latter is illustrated in Fig. 24. 



