_Argument from recent Discoveries._--Indeed, recent discoveries in science seem to vindicate that not only surface extension, but trinal extension, or _solidity_, may be an object of direct perception by the eye. I refer to the researches of Wheatstone, in binocular vision, which go to show, that in consequence of the difference of the images formed upon the right and the left eye, as occupying different positions with reference to the object seen, we are enabled by the eye to cognize the solidity as well as the extension of objects. The difference of figure in the two images gives us this. That such is the case is shown by an instrument, the stereoscope, so constructed as to present separately the image as formed on each eye, which, when separately viewed, appear as mere plane surfaces, but when viewed together, the right image with the right eye, and the left one with the left eye, at the same time, present no longer the appearance of plane surfaces, but the two images combine to form one distinct figure, and that a solid, having length, breadth, thickness, and standing out with all the semblance of the real object. It is hardly necessary to say that if extension is an object of perception by the eye, so also is figure, which is merely the _limitation_ of extension in different directions. _Second Question--Does Sight give Distance?_--Is it also by vision that we obtain the idea of the _distance_ of objects and their externality? Does vision alone give the idea that what we see is numerically distinct from ourselves, and that it occupies this or that particular locality? So it would seem, judging from the impression left upon the mind in the act of vision. We seem to see the object as here or there, external, more or less distant in space. We distinguish it from ourselves. _The negative View._--This is denied by some. All that we see, they contend, is merely the light coming from the object, and from the variations and modifications which this exhibits we learn to judge by experience of the distance and locality of the object. It is a matter of judgment and not of perception. We have learned to associate the two things, the visual appearance and the distance. _Argument in the Negative._--In proof of this they adduce the fact that we are frequently mistaken in our estimate of the distance of objects. If there be more or fewer intervening objects than usual, if the atmosphere be more or less clear than usual, or any like circumstance affords a variation from our ordinary experience, we are misled as to the distance of the object. Hence we mistake the distance of ships at sea, or of objects on a prairie or a desert, the width of rivers, the height of steeples, towers, etc.