Prof. Dove on the Reversion-prism ^ 37 



\'l'Th6 'R^'ii&rdon-prism, and its application as ocular to the Ter- , 

 restrial or Day -Telescope, and to the measurement of angles. 



When rays fall upon the side surface of a prism whose ends 

 are right-angled isosceles triangles^ and parallel to the hypo- 

 thenuse surface of the same, they issue from the other side, after 

 having endured two refractions and one total reflexion. 



An object observed through such a prism appears unaltered 

 in shape and magnitude, but it has changed sides in the same 

 manner as the image of an object in a looking-glass. The con- 

 ditions of achromatism are strictly fulfilled, for all rays which 

 were parallel before their entrance remain so after their exit. If 

 the hypothenuse surface lie horizontal, so that its production 

 shall intersect the object in a horizontal line, then the image is 

 obtained by letting fall from all points of the object perpendi- 

 cularfe upon this line, and producing them until their parts above 

 and below the horizontal line are equal to each other. .The ends 

 of the productions taken all together form the image of the 

 object. 



If therefore a vertical line be intersected in the centre by the 

 above horizontal line, the former line and its image will coincide, 

 its position being reversed. If the line make an angle of 45° 

 with the produced hypothenuse surface, then the image will be 

 perpendicular to the object. Now as an inclination of 45° on 

 the part of the line which was first considered vertical, the prism 

 remaining fixed, has the same efi'ect as if the line remained fixed 

 and the prism were turned 45° in the opposite direction, the 

 image must rotate with double the velocity of the plane of refrac- 

 tion of the rotating prism. 



As the rays which emerge from this first prism with regard 

 to a second similar one may be considered as proceeding direct 

 from an object which occupies the position of the image, it 

 follows, — 



If the hypothenuse surfaces of two equal prisms lie in the 

 same plane, their corresponding edges being parallel, then an 

 object viewed through both will appear altogether unchanged ; 

 for the second prism reverses the image delivered by the first 

 prism, or in other words, it undoes the work of the latter and 

 restores things to their primitive condition. If, on the contraiy, 

 the first prism remain fixed and the second be turned, so that the 

 planes of refraction of both prisms are perpendicular to each 

 other, the object then appears completely reversed. The first 

 prism reverses it with regard to right and left, and the second 

 with regard to top and bottom. As, however, a reversion is 

 equivalent to a turning of the object through an angle of 180°, 

 it follows, — 



