116 Modern Microscopy 



is, T tnr of an inch of the stage micrometer. Perhaps it 

 will be found that there will be several lines of the eyepiece 

 micrometer and a fraction in that space, and in order that 

 this fraction may be obviated the draw-tube should be 

 slightly pulled out, which will give, of course, an increased 

 amplification, until a certain number of the lines on the 

 eyepiece micrometer are exactly equal to a division or 

 divisions on the stage micrometer. We will imagine that 

 the number of eyepiece micrometer lines that fill T <j^ 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 micrometer will still be seen in the 

 field, and bearing in mind that five of these lines equal 

 T tnr of an inch, any part of the object can at once be 

 measured. It must be remembered, however, that with 

 every objective an estimation of the value of the eyepiece 

 micrometer is necessary. 



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 of the object against 

 the micrometer for measurement, especially with high 

 powers. This form of micrometer surmounts this difficulty. 

 The ordinary eyepiece micrometers necessitate no alteration 

 to ordinary eyepieces, but the Jackson form requires that 

 the outer tube of the eyepiece shall be cut to receive the 

 carrier for the micrometer. Fig. 45 shows an eyepiece with 

 the Jackson micrometer, m, in position. 



There is yet another form of eyepiece micrometer, called 

 the Kamsden screw micrometer, which consists of an eye- 

 piece containing two webs or wires, one fixed, the other 

 travelling by means of a screw having 100 threads to the 

 inch. The milled head of this screw is divided into 100 parts. 

 Across the field are very small equidistant V-shaped teeth, the. 

 interval between each of which corresponds to one complete 

 revolution of the milled head. The value of these teeth is 



