2/2 



ACCESSORY APPARATUS 



convenient l)iit important. because the magnification is not uniform 

 throughout 'he field. If the power employed is high, in order to 

 eft'ect tln> .-pan of the great magnification, one wire (the fixed central 

 one) will lit- in the middle of the field, the other at the margin, and 

 the comparison will not lie true on account of the unequal magnifi- 

 cation of the eye-piece throughout the field, whereas if the wire be 

 placed live notches on one side, both measurements are brought more 

 within the centre of the field. 



Messrs. Zeiss now make a Ramsden micrometer eye-piece. It is 

 provided with a gla>s plate with crossed lines, which together with 

 the eve-piece are carried across the image formed by the objective 

 by means of the measuring screw, so that the adjustment always 

 remains in the centre of the field of view. 



FK;. 213. 



Fig. _l:i illustrates this inst riiment, complete and in longitudinal 

 section. 



Kach dixision on t!,e edge of the drum corresponds to 0*002 mm. 

 Wliole turns are counted on a numbered scale seen in the visual 

 field, .-ind the image may be measured up to H mm. 



A modification of this instrument, facilitating both accuracy and 

 >implicity. was in iS'.Mt devised by .Mr. Nelson, 1 of which we think 

 highly, and of which we give an illustration in fig. 214. 



This SCreW micrometer eyepiece differs from tllose of the old 



form mainly in two respects : first, the optical part is compensated ; 

 secondly, the micrometer part uith both webs can be made to 

 traverse en line, the field of the eye-piece by .-crew motion. 



More particularly speaking, the instrument consists oft wo parts : 

 1 Journ. /.'. M.S. IS;M>. ]., 508. 



