EYE-PIECE MICROMETERS. 



233 



and 203, inserted into the ordinary Huyghenian eye-piece. 

 The micrometer itself consists of a series of lines ruled 

 upon glass and set in a frame so that it may be moved 

 forward by a small screw, a spring at the extreme end 

 keeping it always in contact with it. The slits which are 

 cut in each side of the eye-piece 

 should be provided with a small 

 sliding tube to prevent dust 

 entering between the glasses. 



The method of finding the 

 permanent values of the di- 

 visions of this micrometer is 

 identical with that previously 

 described, and when once found with accuracy should be 

 entered in the observer's note-book for future reference. 



FIG. 202. 





FIG. 203. 



Ramsden's screw micrometer, the last we need mention, 

 is shown in Fig. 204. It is composed of a positive eye- 

 piece (see page 42), before which is stretched two very fine 

 parallel wires, one of which can be separated from the 

 other by a screw having one hundred threads to the inch, 

 the head being further graduated into one hundred parts, 

 as shown in the figure representing the instrument made 

 by Mr. Swift. 



The field of view is illustrated by Fig. 205, and shows 

 clearly the lower portion, consisting of a thin plate of 

 brass, the edge being indented with a series of notches ; 

 every fifth indentation is deeper than the others, thus 

 facilitating the counting. Each notch is equivalent to one 



