When two mirrors, therefore, are combined, as in Fig. 14, the eye must be in the plane of both, in order that the object and its image may have a symmetrical coincidence, and therefore it must be at the point =E= where the two planes cut each other. The necessity of this position, and the effects of any considerable deviation from it, will be understood from Fig. 14, where =A O B= is the angle formed by the mirrors, and =M N= the place of the object. Then if the eye is placed at ε, the aperture =A O B= will be projected into =_a b_ ω= upon a plane passing through =M N= and at right angles to =E Oʹ=; but the orthographic projection of =A B O= upon the same plane is =Aʹ Bʹ Oʹ=, or, what is the same thing, the reflecting surfaces of which =A O=, =B O= are sections, will, when prolonged, cut the plane passing through =M N= in the lines =Aʹ Oʹ=, =Bʹ Oʹ=; hence, rays from the objects situated between =Aʹ Oʹ Bʹ= and =_a_ ω _b_= cannot fall upon the mirrors =A O E=, =B O E=, or images of these objects cannot be formed by the mirrors. The images, therefore, in the different sectors formed by reflexion round =O= as a centre, cannot include any objects without =Aʹ Oʹ Bʹ=; and since the eye at ε sees all the objects between =Aʹ Oʹ Bʹ= and =_a_ ω _b_=, there can be no symmetry and uniformity in the picture formed by the combination of such an object with the images in the sectors. When the eye descends to _e_, the aperture =A O B= is projected into =_aʹ oʹ bʹ_=, which approaches nearer to =A O B=; but for the reasons already assigned, the symmetry of the picture is still imperfect. As the eye descends, the lines =_aʹ oʹ_, _bʹ oʹ_= approach to =Aʹ Oʹ=, =Bʹ Oʹ=, and when the eye arrives at =E=, a point in the plane of both the reflecting surfaces, the projection of the aperture =A O B= will be =Aʹ Oʹ Bʹ=, and the images in all the sectors will be exactly similar to the object presented to the aperture. Hence we conclude in general, _that when an object is placed at any distance before two mirrors inclined at an angle, which is an even aliquot part of 360°, the symmetry of the picture is perfect, when the eye, considered as a mathematical point, is placed at_ =E=, and that _the deviation from symmetry increases as the eye recedes from_ =E= _towards_ =ε=. If the object were a mathematical surface, all the parts of which were in contact with the extremities =A O=, =B O= of the mirrors, then it is easy to see that the symmetry of the picture will not be affected by the deviation of the eye from the point =E=, and, in consequence of the enlargement of the sector, seen by direct vision. The symmetry of the picture, is, however, affected in another way, by the deviation of the eye from the point =E=.