THE introduction of the stereoscope inaugurated a new epoch in the physiology of vision, opened a wide field for further inquiry, and suggested additional methods of investigation, while the theory of binocular vision has been greatly modified by results which have been obtained through the medium of this instrument.

The stereoscopic theory of vision is not, however, free from difficulty, since it implies a remarkable exception to a general law, for it is based upon the assumption that two sensations occasion one resultant perception, although in no instance can it be shown that two nerves terminate in one centre.

The experiments adduced by Professor Wheatstone are very striking, but their value for what they have been supposed to prove turns upon the question, are the results which they exhibit to be referred to mental construction or to structural arrangement? Professor Wheatstone, to whom we are indebted for the stereoscope, refers them to the mind, and his views are explained in a communication entitled “Contributions to the