ABSTRACT
When the observed objects are known to us, the angles they subtend, or, rather, the spaces which their images cover upon the retina, aid in the estimate. The particular focal adjustments which the eyes must undergo to obtain distinct vision, and which are accompanied by certain muscular sensations, further assist. And the muscular sensations accompanying the due convergence of the visual axes, supply a third evidence. Not only between the elements of each cognition, and between the elements of each operation, does this connection grow ever more close; but also between cognitions and the operations guided by them. Being a process through which only, highly special and complex correspondence can be achieved, progress in integration has been a necessary accompaniment of progress in speciality and complexity ; and in proportion as civilization has displayed the last, it must have displayed the first.
