73 
*ld- With movable index in lieu of micrometer scale . . 
Screw Micrometer Eye-piece, f or such exact meas- 
ure m e n t s of objects which occupy a large portion 
of the field and cannot, therefore, be measured with 
sufficient accuracy by means of the ordinary micro- 
meter eye -piece. Ramsden eye-piece; glass-plate with crossed 
lines which are moved across the field by means of the micro- 
meter screw. — Each interval of the divided drum corresponds 
to 0.01 millimeters of the objective image. Complete revolutions 
of the drum are counted by means of a figured scale in the field. 
With the aid of this instrument up to 8 mm may be measured 
across the image formed by the objective. (Fig. 32.) .... 
Id. fitted with compensating eye-piece in lieu of the 
Ramsden eye-piece ; for use with apochromatic lenses .... 
Stage Screw Micrometer, for the exact measure- 
ment of objects too large to be included in one visual 
Fig. 33. Stage Screw Micrometer. 
