W. B. Rogers on Binocular Vision. 217 
26. 26a. 
DOP@ 
the relief of the image. When this is done by fixing the view 
ou the base, the resultant is the plane figure represented in fig. 
26a, in the other case the figure is the converse of this. 
It may be added than among the linear drawings common! 
used with the stereoscope there are some in which these effects 
are less observable than in the above instances, and others in 
which the attempt to obtain a clear view of either the nearer or 
verged, it may be laid down asa rule in the selection of such 
figures for the stereoscope that the most satisfactory drawings are 
those in which there is no anterior line crossing a posterior one 
and in which when two such lines are parallel they are not situa- 
ted in or near the same vertical plane of view. 
_ Asa very different degree of convergence is required for uni- 
ting the lines proper to the near side of the perspective image 
from that necessary for the remote side, more or less time must be 
consumed in passing from the one to the other. Hence in many 
cases even the most practised observer is unable to make this 
transition with sufficient celerity to have a clear perspective im- 
pression of the whole, and to this cause is no doubt due in part, 
the difficulty experienced by most persons unused to the stereo- 
Scope, in embracing distinctly the form and relief of the entire 
Image 
