The measuring arm A (Fig. 1) pivots about a point B created by a single 
captive, top-loaded, self-seating ball bearing having a diameter of 0.75 inches. 
The micrometer end C of the measuring arm is supported by a pair of rollers that 
run on the guide rail D. The measuring arm can be locked at any desired point 
on the guiderail by releasing either of the dual, spring loaded brake levers E. 
A closeup of the measuring arm with its side panels removed is shown in 
Fig. 3. A Bausch and Lomb zoom macroscope (10X^ 30X)F is mounted on a carriage 
G which rolls on sleeve bearings along a pair of stainless steel rods and can be locked 
at any desired point by releasing the spring loaded brake lever H. 
The scale | is 6mm thick, 29mm wide and 292mm long. The bottom side is 
graduated at one millimeter intervals over a length of 261mm. Every even division 
is numbered. The ends of the scale are attached to supports which ride on sleeve 
bearings on a pair of stainless steel rods. The lower support of the scale is free and 
the upper support is attached to the nut of the micrometer screw which is two 
centimeters in length. By turning the micrometer drum J, one can translate the 
scale radially from the pivot by a precisely known amount. Automatic stops in the 
micrometer head limit this translation to at most one millimeter. The amount of 
translation can be read by a vernier to the nearest half micron. 
The plate to be measured is mounted in the plate holder K, the four corners 
of which rest freely on four pads machined to be in the same plane as the upper 
guiderail. Different plate holders are made to accommodate plates having the 
following nominal dimensions: 9.5x 9.5 inches, 8x 10 inches, 190x 215mm. 
Each plate holder is pre-adjusted to accommodate a standard plate thickness of 
.240 inches. This leaves a gap of 75 to 100 microns between the upper surface of 
the plate and the bottom surface of the scale. Such a gap is sufficiently small to 
permit both the plate and the graduations of the scale to be sharply in focus at the 
upper magnification of 30X. Standard plates thinner than .240 inches can be 
accommodated by means of thumbscrew adjustments in the corners of the plate 
holder. On each edge of the plateholder a pair of V-blocks that have been mated 
to a pair of positioning pins permanently affixed to the mainframe serve to fix the 
plateholder in each of the four standard measuring positions. 
Illumination of the plate is provided by a built-i light table containing 
three 8 watt fluorescent lamps. A pair of folding legs built into the back of the 
mainframe support the comparator in its normal operating position at an angle of 
45" from the horizontal. 
Because the stainless steel rods of the measuring arm were selected to have 
very nearly the same coefficient of expansion as glass, the comparator is relatively 
insensitive to variations in temperature. The glass employed for the scale is of the 
same type as is used for Kodak plates. 
OPERATION OF THE COMPARATOR 
By rotating the measuring arm and translating the microscope, one can 
quickly bring any desired point on the plate into the field of the microscope. To 
measure a point, one first brings it within a circular reticle having a diameter of 
400 microns at plate scale. There is no need for precise centering of the image 
“+ ~~ 0 3 
= — 0 Q 3 
Mm