There is another variation of these experiments which possesses some interest, in consequence of its extreme case having been made the basis of a new theory of visible direction, by the late Dr. Wells.[39] Let us suppose the eyes of the observer to advance from E to N, and to descend along the opposite quadrant on the left hand of NG, but not drawn in Fig. 27, then the united image of AC, BC will gradually descend towards CG, and become larger and larger. When the eyes are a very little above the plane of ABC, and so far to the left hand of AB that CA points nearly to the left eye and CB to the right eye, then we have the circumstances under which Dr. Wells made the following experiment:—“If we hold two thin rules in such a manner that their sharp edges (AC, BC in Fig. 27) shall be in the optic axes, one in each, or rather a little below them, _the two edges will be seen united in the common axis_, (GC in Fig. 27;) and this apparent edge will seem of the same length with that of either of the real edges, when seen alone by the eye in the axis of which it is placed.” This experiment, it will be seen, is the same with that of Dr. Smith, with this difference only, that the points of the compasses are directed towards the eyes. Like Dr. Smith, Dr. Wells has omitted to notice that the united image rises above GH, and he commits the opposite error of Dr. Smith, in making the length of the united image too short. [38] Smith’s _Opticks_, vol. ii. p. 388, § 977. [39] _Essay on Single Vision, &c._, p. 44. If in this form of the experiment we fix the binocular centre beyond C, then the united images of AC, and BC descend below GC, and vary in their length, and in their inclination to GC, according to the height of the eye above the plane of ABC, and its distance from AB. CHAPTER VII. DESCRIPTION OF DIFFERENT STEREOSCOPES. Although the lenticular stereoscope has every advantage that such an instrument can possess, whether it is wanted for experiments on binocular vision—for assisting the artist by the reproduction of objects in relief, or for the purposes of amusement and instruction, yet there are other forms of it which have particular properties, and which may be constructed without the aid of the optician, and of materials within the reach of the humblest inquirers. The first of these is— 1. _The Tubular Reflecting Stereoscope._ In this form of the instrument, shewn in Fig. 28, the pictures are seen by reflexion from two specula or prisms placed at an angle of 90°, as in Mr. Wheatstone’s instrument. In other respects the two instruments are essentially different.