52 
ACCESSORY APPARATUS. 
To illustrate the method of making these calculations, we 
give the process and results in a single case, using the ■§■ inch 
objective and negative eye-piece [N’o. 2 , in which is inserted a 
glass micrometer, with lines ruled about 2 Jo inch; (the 
exact value of these lines is of no conse(][uence, as their value, 
as used for micrometer measurements, depends on the magnify- 
ing power of the glasses used.) We select a stage micrometer 
with lines ruled at of an inch, with no covering over the 
lines. We place this micrometer on the stage with the ruled 
lines upward. Setting the adjustment of the object-glass at the 
mark ^ uncovered^ and carefully adjusting the focus, we find 
one space on the stage micrometer covers very nearly nineteen 
spaces in the eye-piece micrometer ; we therefore increase the 
magnifying power, by extending the draw-tube, till a conven- 
ient number, as twenty spaces, are covered by one space on the 
stage, when we find that the draw-tube is extended xVo 
inch. 
As now twenty spaces in the eye-piece micrometer are equal 
to 5 Jo an inch on the stage, each division in the eye-piece 
measures toJw of an inch on the stage, when the object is 
uncovered, and the draw-tube is extended of an inch. We 
find this estimate exceedingly convenient for use, and accord- 
ingly record the conditions and estimated measurement for 
future reference. 
But as our measurement will often, perhaps generally, be 
made upon objects covered with thin glass, we now place 
a cover of thin glass over the micrometer lines on the stage, 
and repeat our calculations. We correct the object-glass for 
thickness of glass cover, by turning the graduated collar till we 
obtain perfect definition of the lines on the stage micrometer, 
and we find, in this case, that the adjustment of the object- 
glass has been turned forward twelve and one-half degrees of 
its graduated scale ; counting the spaces in the eye-piece 
micrometer, covered by one space on the stage, we find, not 
twenty as before, but twenty-one and a fraction; we therefore 
diminish the magnifying power by pushing back the draw-tube 
till twenty spaces in the eye-piece again exactly fill one division 
J. & W. GRUNOW & GO’S ILLUSTRATED 
