GV-60 PHOTOGRAMMETRIC ENGINEERING 
the development time can be increased to a maximum of 21 minutes, at which 
point the fog level begins to rise noticeably. With Weston readings of 800 or 
over, the development time should be kept at 10 or 11 minutes. Of course, if 
the light reading falls to 75 Weston, the diaphragm may be opened to f:8, and 
if the reading is as low as Weston 50 it is generally necessary to use an opening 
of 1.6.3. 
THE TIMER 
The timer used in the analyzer includes a three-wheel digital counter and a 
rotating drum marked with ten helical lines visible through a slit, a lamp to 
illuminate the timer face, and a constant speed motor to drive the mechanism. 
The digital counter may be read directly in seconds, tenths of seconds, and 
hundredths of seconds. Still smaller time divisions are recorded by the rotating 
helical lines, visible through the viewing slit as white dashes which move from 
the bottom to the top of the slot in 0.01 second. Markings behind the slit make it 
easy to read to thousandths of a second because the 1/6,000th second exposure 
freezes the dash and yields an image of great clarity. 
The unit is driven by a 6-volt D.C. motor, accurately controlled by a 
vibrating tuned reed to a speed of 900 R.P.M. 
TRACKING MECHANISM 
Tracking is accomplished by manually swinging the crossbar that extends 
across the top of the camera parallel to the focal plane. Tracking with the 
analyzer has been facilitated by the incorporation of an automatic pick-up 
and release mechanism that enables the operator to track through 74 degrees 
before the shutter mechanism becomes operative, and to track through a similar 
angular distance after the shutter has crossed the plate and has ceased to oper- 
ate. This lead-in and follow-out tracking of the target before and after recording 
permits the operator to find the target and to accommodate his tracking speed 
to it before any exposures are made. The pick-up and release mechanism oper- 
ates in either direction, from left to right, or from right to left. 
Another device calculated to aid in tracking consists of the weights attached 
to the tracking arms. The use of inertia weights was decided upon after ex- 
perimentation with other inertia devices, such as flywheels and friction slip 
clutches, because inertia weights offered the most flexible and versatile method 
of smoothing out the tracking process. If a particular test requires very rapid 
tracking acceleration, for example, the weights can be omitted entirely or smaller 
weights used instead of heavy ones. Similarly it is possible to use shorter track- 
ing arms for certain types of tests. 
The conversion of the rotary motion of the tracking head into the linear 
motion of the traversing shutter is done by the use of a tangent bar, slotted on 
its rearward end to engage a pin that actuates the shutter-moving mechanism. 
The tangent bar moves in an arc with the tracking head; and as this bar moves, 
the pin in its slot (and the entire shutter mechanism) moves laterally in a path 
parallel to the focal plane. The tangent bar and the slotted pin mechanically 
perform the trigonometric corrections necessary to convert angular motion to 
constant scale linear motion. Thus the progression of images on an analyzer 
plate form an accurate reproduction of the forward motion of the target, and 
mathematical trigonometric correction of the values on the plate is unnecessary. 
TRACKING OPTICAL SYSTEM 
The target is tracked visually through 6X30 binoculars (10X50 on the 
Model VI analyzer) equipped with a reticle showing two vertical lines represent-