the micrometer so that one of its .lines shall exactly 

 coincide with one end of the drawing on the paper and 

 then measure off how many spaces are covered by 

 the object. Thus if o.ooi inch is the value of the 

 micrometer spaces and the object covers one space, 

 its size will be o.ooi inch, or covering seven spaces 

 will be 0.007 m ch. 



A variation of the distance of the camera lucida 

 from the paper, or a change of power in eyepiece or 

 objective does not vary the results so long as object 

 and micrometer are used under exactly the same 

 conditions. 



The second method is with the eyepiece micrometer 

 or micrometer eyepiece. The eyepiece micrometer 

 consists of a circular disc of glass of suitable size to 

 just fit inside the tube of the eyepiece, resting upon the 

 diaphragm at the focus of the eye lens, scale side up, 

 or mounted in an oblong holder to be slipped into a 

 slot in the eyepiece mounting just above the diaphragm. 

 In case the lines on the eyepiece micrometer do not 

 show plainly, adjust the eye lens until they do. The 

 micrometer eyepiece, Fig. 45, in which the eyepiece 

 and micrometer form a complete apparatus and a 

 lateral adjustment of the scale across the field is given 

 by a screw, is much more convenient. 



In either of these forms the ruled lines appear to 

 lie directly on the image of the object, but while we 



