and on our Perception of Distance. 371 
Our normal vision being binocular, the distance between the 
optical centres of the two eyes, becomes a base line of constant 
length, to which all external distances are referred, with a pre- 
cision, constantly decreasing, both outwards and inwards, from 
au 98 h 
judgment, to those of more remote objects, where the direct sense 
of distance fails us. Here too, as for near objects, we doubtless 
involuntarily comes to our aid. Our knowledge of the ordinary 
and absolute sizes of the various classes of objects seen, will be- 
sides greatly affect our rational estimates of distance. Thus sen- 
estimates. 
If these views be correct, monocular vision should be much 
less accurate than binocular in the estimation of distances near at 
The lack of the interocular base would make the process 
of perspective gaugeing almost exclusively an act of judgment. 
The only sensational element in the former, would be that of a 
conscious focalization of the eye, which could hardly be of much 
importance. Thus, I should conclude, that persons with only a 
single effective eye, would be bad judges of distance, especially 
among near objects. How the fact stands, I have had no means 
of ascertaining. 
