171 



in the least disturbing- any of the conditions of 

 tube length or distance from the paper. Remove 

 the object and replace it by the stage micrometer, 

 focusing only with the line adjustment and, it may 

 be added, there should be very little variation in 

 thickness between the two slides. Move the micro- 

 meter so that one of its lines shall exactlv coincide 



/ 



with one end of the drawing- on the paper and then 

 measure off how many spaces are covered by the 

 object. Thus if 0.001 inch are the values of the 

 micrometer spaces and the object covers one space, 

 its size will be n.nni inch, or covering ~ spaces will 

 be 0.007 inch. 



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 result so long- as 

 objective and micrometer are used under exactly 

 the same conditions. 



The second method is with the eyepiece micro- 

 meter which in its simple form is a micrometer 

 mounted on a plate which is placed in the focus 

 of the eye lens. It is usually graduated to 0.1 mm., 

 seldom exceeding 0.05 mm., as closer lines cannot 

 well be distinguished with the magnification 

 obtained by the eye lens. The plate is dropped 

 onto the diaphragm, where it is in focus, or enters 

 slots which are provided in the tube of the eye- 

 piece. A better form however, is the eyepiece 

 micrometer (Fig. 4.">) in which the eyepiece and 



