On the other hand if a second prism be placed behind the first, flat upon its side, with its hypothenuse face occupying a plane exactly perpendicular to that of the first prism, the line A′B′ will be refracted, totally reflected and refracted again out of the prism without a second inversion, while a line perpendicular to A′B′ will be refracted endwise on the hypothenuse face of the second prism and will be inverted as was the line A B at the start.
Fig. 113.—Reversion Prism.
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.
Fig. 114.—Dove’s Prisms.
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.
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.