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SUMMARY OF CURRENT RESEARCHES RELATING TO 
in a metal frame, which could be inserted in a slit in the eye-piece, 
which was a negative one. 
Of the micrometer eye-pieces with movable scales, the author de- 
scribes that of Ramsden, which was first used for telescopes. It consists 
of a positive eye-piece, in the focus of which two parallel wires or 
cobwebs are stretched across the field. One of these is fixed, while the 
other can be moved by means of a screw provided with a divided head. 
Entire revolutions of the screw are registered by a metal comb on the 
lower side of the field. 
Nelson has modified this micrometer so that both wires can be 
moved en bloc across the field. 
Lastly, photomicrography supplies an accurate means of measuring 
objects. The image of the stage micrometer is thrown upon the ground 
glass, and then can be measured or photographed, as desired. 
Zeiss’ Stage Screw Micrometer. " — This apparatus, shown in fig. 50, 
is intended for the exact measurement of objects too large to be included 
Fig 50. 
in one visual field of the Microscope. A sliding-piece actuated by a 
micrometer-screw, carries a rotating disc with divided circle, the divisions 
of which indicate 0* 002 mm. The screw measures up to 10 mm. 
* Zeiss’ Catalogue, No. 30, 1895, pp. 75-6. 
