a distant surface, with which I necessarily identify it as belonging to its effect, the cross will occupy 2° of a distant and therefore larger sphere, and is consequently large. If, on the other hand, I project the image on to a nearer object, it will occupy 2° of a smaller sphere, and is therefore small. The resulting perception is in both cases completely objective, quite like that of an external object; and as it proceeds from an entirely subjective reason (from the image having been excited in quite a different way), it thus confirms the intellectual character of all objective perception.—This phenomenon (which I distinctly remember to have been the first to notice, in 1815) forms the theme of an essay by Séguin, published in the "Comptes rendus" of the 2nd August, 1858, where it is served up as a new discovery, all sorts of absurd and distorted explanations of it being given. Messieurs les illustres confrères let pass no opportunity for heaping experiment upon experiment, the more complicated the better. Expérience! is their watchword; yet how rarely do we meet with any sound, genuine reflection upon the phenomena observed! Expérience! expérience! followed by twaddle.
To return to the subsidiary data which act as commentaries to a given visual angle, we find foremost among them the mutationes oculi internæ, by means of which the eye adapts its refractory apparatus to various distances by increasing and diminishing the refraction. In what these modifications consist, has not yet been clearly ascertained. They have been sought in the increased convexity, now of the cornea, now of the crystalline lens; but the latest theory seems to me the most probable one, according to which the lens is moved backwards for distant vision and forwards for near vision, lateral pressure, in the latter case, giving it increased protuberance ; so that the process would exactly resemble the mechanism of an opera-glass.
