Parallax measurements and point transfers are made without confusing shadows by 
using the transmitted light, and the longer tone range is available. This is definitely 
a laboratory instrument, but it does enable the operator to see what the pictures contain, 
with the minimum effort or distraction. The extensive and the intensive examinations 
are available in the same photographs. The complete overlap can be seen at once, or 
can be scanned at 3 or 8 magnifications. These instruments come close to doing 
modern photography justice. (See Fig. 4.) 
  
Fig. 4 
Mirror stereoscope with binoculars and accessories 
Summary 
To obtain maximum interpretability of the photograph, then, the following arc 
necessary. 
1. Obtain maximum sharpness in the negative. 
2. Obtain optimum densities and contrasts in the positive. 
3. Obtain adequate visual acuity and depth of fusion in the stereoscope. 
Field Tests 
Three years ago, the Ontario Department of Lands and Forests asked me to rewrite 
the specifications for the aerial photography used for Forest Inventory. 
In accepting this assignment I requested the initiation of a field test programme to 
determine in a quantitative manner the relative merits of available cameras, lenses and 
films, and processing and printing techniques, in order to confirm or correct the 
specifications as written. 
Last year (1958) we constructed at the Lands and Forests research station near 
Maple, Ontario, two level platforms, one 160 X 16 ft. and the other 144 X 16 ft., about 2 ft. 
above the ground (see Figs. 5 and 6). The 160 ft. platform supports a "grey scale", 
consisting of 9 squares, 16 X 16 ft., of 1/2 inch plvwood painted with matt paints of 
specified reflectances from .90 to .04 (which provides a means of assessing the degrada- 
tion of contrast by haze), and a “black box”, 16 X 16 X 1 ft. deep, subdivided into 2034 
cells, each 4 X 4 X 12 inches deep, with thin aluminum walls painted glossy black on a 
matt black base, which constitutes an area of very low reflectance and provides both a 
means of assessing the photographic rendering of haze and a “zero brightness” back- 
ground for the special-purpose targets described below. 
12