54 
ACCESSORY APPARATUS. 
inch ; which is a degree of accuracy sufficient for all ordinary 
observations. Those who desire more accurate measurements 
will, of course, procure the more expensive instruments. For 
more detailed descriptions of the method of using microme- 
ters, we would refer our readers to the valuable works of 
Quecket and Carpenter on the microscope. 
73. Frainihofer’s Stage-§crew-Micro]iieter. The labor 
of making the necessary calculations to determine the value of 
measurements made by micrometers placed in the eye-piece of 
the microscope, and the amount of care required to secure 
accurate results, have rendered it desirable to obtain a microm- 
eter at once minutely accurate, simple in its use, and requiring 
no calculation of the value of its measurements. 
The instrument most nearly fulfilling these requirements 
is Fraunhofer’s Stage-Screw-Micrometer. The value of the 
measurements made by this micrometer is always independent 
of the magnifying power of the microscope. The instrument 
indicates, directly, and with great accuracy, the absolute 
dimensions of the object measured. 
This micrometer is placed upon the stage and consists essen- 
tially of two plates, one of which, carrying the object, is 
moved upon the other by a micrometer-screw, having one 
hundred threads to an inch, with a graduated head and vernier 
by which motions of the plates and object upon it are accu- 
rately measured to the one hundred thousandth of an inch. 
Fig. 24 shows this instrument, the upper part of the figure 
being a view from above, while the lower part of the figure 
gives a side view of the same. 
B B is the body of the instrument which is fastened to the 
stage of the microscope by the short cylinder C, which is made 
elastic so as to hold the instrument securely in its place when 
it is inserted into the circular opening of the stage. A A 
is a revolving plate with spring clips to support the object to 
be measured. This plate can be revolved so as to bring the 
object in a position to measure it in any direction. This circu- 
lar plate is supported on another plate in the body of the 
instrument, which is moved by a fine micrometer screw, with 
exactly one hundred threads to the American standard inch. 
.J. & W. GRUNOW & GO’S ILLUSTRATED 
