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ACCESSORY APPARATUS 



convenient but important, because the magnification is not uniform 

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

 effect the span of the great magnification, one wire (the fixed central 

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

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

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

 placed five 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 glass plate with crossed lines, which together with 

 the eye-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. 



FIG. 213. 



Fig. 213 illustrates this instrument, complete and in longitudinal 

 section. 



Each division on the edge of the drum corresponds to 0*002 mm. 

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

 field, and the image may be measured up to 8 mm. 



A modification of this instrument, facilitating both accuracy and 

 simplicity, was in 1890 devised by Mr. Nelson, 1 of which we think 

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



This screw micrometer eye-piece differs from those of the old 

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

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

 traverse en bloc the field of the eye-piece by screw motion. 



More particularly speaking, the instrument consists of two parts : 

 1 Journ. B. M. S. 1890, p : 508. 



