ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
239 
increasing pull of the lever. The extension of the fibre is measured by 
the movement of the scale D across the field of the Microscope, and the 
deflection of the lever B is a measure of the force that is being applied 
to the fibre, which is obtained by subtracting the amount of extension of 
the filament from the distance traversed by the Microscope, which latter 
may be determined with the greatest accuracy by the readings of the 
micrometer-head. 
In adjusting the instrument, the slide is first made vertical by 
levelling screws, the accuracy of the levelling being determined by 
means of a spirit-level placed in different azimuths on the to]) of the 
micrometer-head. The counterweight P is next adjusted until the 
knife-edges at a and h are 
both in the same horizontal 
plane, and this adjustment 
is made when the scale D 
and the upper attachment 
pin of the fibre are in their 
proper position, and the 
Microscope is focused so 
as to give a sharp definition 
of the divisions on the 
scale. The fibre having 
been attached to the upper 
supporting pin and sus- 
pended in its place, the 
length of the arm A is so 
adjusted that the lower 
supporting pin of the fibre 
hangs freely in the axis of 
the clamp C, which is then 
tightened, and thus perfect 
verticality at the com- 
mencement of the pull is 
insured. The micrometer-head is then slowly turned, readings being 
taken as each division of the scale D traverses and coincides with the 
cross wire of the Microscope, and the force which thus extends the fibre 
by each increment of 1/20 mm. is determined in the following way. 
If the adjustments of the instrument have been made in the manner 
described above, the moment due to a vertical pull is proportional to 
the cosine of the angular displacement of the beam, while that due to 
the gravity-bob and the other portions of the beam varies as the sine of 
that angle, the actual tensile force applied at D being proportional to 
the tangent of the inclination of the beam. The vertical distance c 6 is 
a measure of the sine of the inclination, and when the angular displace- 
ment is small this distance is practically the same as the tangent of the 
angle, and it may be corrected to measure the tangent if very great 
accuracy be reiiuired. The true value of the force corresponding to 
various values of c 6 may, however, be more easily found by attaching 
weights to D, and observing by means of the Microscope and scale the 
weights which produce com spending deflections. 
In this instrument there are two appareiit sources of error, which, 
Fig. 25. 
