Fraunhofer’s stage-screw-micrometer. 
55 
Fig. 24. 
On the end of this screw is a milled head m, with a head 
having one hundred divisions ; by means of the vernier ti, each 
division of the scale g is divided into ten parts, each of which 
is equivalent to one thousandth part of a turn of the screw. 
The graduated head g can be turned without moving the milled 
head m or the screw to which it is attached, by which means 
the graduation can be set at zero in any position of the screw 
and object. At -y is a scale showing the number of turns 
given to the screw, each equal to one hundredth of an inch, 
therefore the divisions of g are ten thousandths, and the divi- 
sions shown by the vernier are hundred thousandths of an inch, 
whatever may be the magnifying power of the microscope 
by which the object is viewed. 
Accompanying this micrometer is a negative eye-piece with 
a cobweb drawn across the field of view, in the focus of 
the eye-lens, the magnitude of an object being determined 
CATALOGUE OF ACHROMATIC MICROSCOPES. 
