On p63 of the Book, there is a description of how to use special binoculars to find the Distance of an object, if one know its height or width [Real Size]

Distance equals Size times 1000 divided by the reading on the scale [Apparent Size].

In this case,the Binoculars have been made so that the Apparent Distance, relative to the scale, is 1000.

So, using terms Real Distance [D]; Real Size {S}; Apparent Distance [d]; Apparent Size {s}, we have the Ratios **D:S::d:s**, this becomes D:: Sd:s, or D=Sd/s; thus Real Distance is in inverse proportion to Apparent Size.

If one has no special device for Range Finding. he may go primitive: for Apparent Distance [d] use the length from the eye to the scale that is held by the outstretched arm [usually 10 time the length of the thumb, or the distance between the eyes: see *Stereoscopic Ranging* p169]. Thus for the scale, hold up a Ruled Edge to the Distant Object, invert the figure as a fraction of the units[^{1}/_{s}], multiply by Real Size{S} & Apparent Distance[d]. The result will be the Real Distance [D].

NOTE: NO measurement is exact, especially with things that move.