2 9 
The fliie adjustment. This also lias during the last years received our 
special attention. The result of our efforts is the *Micro meter movement 
of new construction now fitted in all our stands except No. IX. (For detailed 
description see Zeitschrift fiir wissenschaftl. Mikroskopie, III, p. 207, 1886.) The 
superiority of this new arrangement is mainly due to the force exercised by the 
micrometer screw being transferred to the moveable limb by a single contact 
between hardened steel surfaces. This ensures an extremely delicate and uniform 
motion of the limb carrying the tube. 
The divisions on the milled head of the micrometer screw in stands 
I — IV furnish a means for exactly registering the vertical movements of the tube. 
In the new stands each division corresponds to 0.01 -mm elevation or depression 
of the tube in the direction of the optic axis. 
By this means measurements of thickness may be made with some 
degree of accuracy. The upper and lower surfaces of the object are successively 
focussed and the amount read off on the milled head by the fixed index. In doing 
this it must be remembered to make both adjustments by a rotation of the screw 
in the same direction. The depth of layers of air is then equal to the differ- 
ence between the two readings. 
The thickness of layers of any other substances may also be measured by the 
same arrangement. Estimation of the thickness of cover-glass for instance is best 
done as follows: With a medium-power dry lens (D or E) and eye-piece 3 or 4, 
using central illumination focus the upper and lower surfaces of cover-glasses of exactly 
known thickness — e. g. the covers of an Abbe test-plate — and note the appar- 
ent thickness so obtained. A comparison of this with the known true value 
gives, once for all, the coefficient for reducing measurements, made with the 
same objective under precisely similar conditions, of any other covers to their true 
values. Roughly speaking this equals 3/2 or more exactly 1.52 (the refractive index 
of glass). The thickness of sections is estimated in a similar manner. 
The medium tube length of our stands is 160 mm from the attachment 
of the objective to the upper end. 
The draw-tube arrangement which is possessed by all stands with the ex- 
ception of stand IX and the small stand for mineralogical purposes, admits of the 
tube-length being increased or diminished. With stands I to IV the length of 
the tube may be read off by means of a millimeter scale worked upon the tube. 
The lower end is tapped with the standard thread to take the auxiliary objective 
used with the apertometer. 
The inner diameter of our draw-tubes is 23 mm at the upper end. 
