and Vision by Optic Divergence. 359 
eyes to replace the photographer’s cameras, and the convergence 
of his visual axes to replace that of axes from some point in the 
landscape upon which these cameras have been directed. In fig. 
1 let aa’ be the fore- 
ground interval and 
bb’ that for the back- 
ground appears at A 
and the background 
at B 
To determine the 
apparent distance of 
A, let i stand for the observer’s interocular distance, RL; a for 
the optic angle, RAL, and D for the apparent distance required. 
Then, if a@ and a’ be symmetrical, . 
, ¥ 
such 
cases the formula just deduced is applicable with little or no 
modification. If 7 stand for the distance between two photo- 
grapher’s cameras directed to the same point in a landscape, 
the formula is also applicable to them, provided there be no lack 
of uniformity in the media through which the rays pass. In 
normal vision, moreover, both the focal and axial adjustments 
of the eyes are consensually adapted to the distance of the o 
ject regarded, and the deliverances of the muscular sense from 
the ciliary and rectus muscles conduce to the same judgment of 
distance,” This judgment is the product of the past experience 
of the individual, and its accuracy must depend largely upon 
his acquired skill in interpreting muscular sensations, compar- 
