This communication was chiefly directed to the question whether the idea of distance be obtained from the adjustment of the eyes to distinct vision, or from the convergence of their axes. The case of the chameleon was cited as one in point, since that lizard directs its eyes each to a separate object, but habitually, when about to strike its prey, brings both eyes to bear upon it. Several experiments, mostly suggested by Wheatsone's inquiries, were cited, and the conclusion was arrived at, that, although the adjustment for direct vision concur in the formation of the estimate of distance, the convergence of the eyes plays the principal part.