Consequently two prisms thus placed will completely invert the image, producing exactly the same effect as the ordinary inverting system Fig. 106. The simple reversion prism is useful as furnishing a means, when placed over an eye lens, and rotated, of revolving the image on itself, a procedure occasionally convenient, especially in stellar photometry. The two prisms together constitute a true inverting system and have been utilized in that function, but they give a rather small angular field and have never come into a material amount of use. The exact effect of this combination, known historically as Dove’s prisms, is shown plainly in Fig. 114. The first actual prismatic inverting system was due to M. Porro, who invented it about the middle of the last century, and later brought it out commercially under the name of “Lunette à Napoleon Troisiéme,” as a glass for military purposes. [Illustration: FIG. 114.—Dove’s Prisms.] [Illustration: FIG. 115.—Porro’s Prism System.] The prism system of this striking form of instrument is shown in Fig. 115. It was composed of three right angle prisms _A_, _B_, and _C_. _A_ presented a cathetus face to the objective and _B_ a cathetus face to the ocular. Obviously a vertical element brought in along _a_ from the objective would be reflected at the hypothenuse face _b_, to a position at right angles to the original one, would enter the hypothenuse face of _C_ and thence after two reflections at _c_ and _d_ flatwise and without change of direction would emerge, enter the lower cathetus face of _B_ and by reflection at the hypothenuse face _e_ of _B_ would be turned another 90° making a complete reversion as regards up and down at the eye placed at _f_. An element initially at right angles to the one just considered would enter _A_, be reflected flatwise, in the faces of _C_ be twice reflected endwise, thereby completely inverting it, and would again be reflected flatwise from the hypothenuse face of _C_, thereby effecting, as the path of the rays indicated plainly shows, a complete inversion of the image. Focussing was very simply attained by a screw motion affecting the prism _C_ and the whole affair was in a small flat case, the external appearance and size of which is indicated in Fig. 116. [Illustration: FIG. 116.—Lunette à Napoleon Troisiéme.] [Illustration: FIG. 117.—Porro’s First Form of Prisms.]