Prof, Dove on several Prisni'Stereoscopes. 31 



eye. If the drawing represents a body, whicli, like a pyramid 

 or the frustum of a pyramid, appears with regard to the surface 

 of the paper either as convex or concave rehef, then the said 

 drawing viewed through the prism with one eye, and with the 

 other eye naked, will appear in convex relief; when the prism is 

 held before the other eye, the stereoscopic combination of both 

 images gives rise to a concave relief. If the drawing be turned 

 in its own plane while the prism remains before the same eye, a 

 rotation of 90° causes the coincidence of the images to appear as 

 a plane projection, A rotation of 180°, on the contrary, changes 

 it from concave to convex. During the rotation, the hypo- 

 thenuse surface must stand perpendicular, or nearly so, to the 

 observed drawing. 



With regard to the dimensions of the prism for holding 

 freely in the hand, an inch in length for the two equal sides of 

 the isosceles triangle at the end, and three-quarters of an inch 

 in width, will be found convenient proportions. When fastened 

 in a cylindi'ical tube attached to a stand, which permits of its 

 being raised and lowered, a prism will be found sufficiently large 

 in which the height of the right-angled triangle does not quite 

 amount to two lines. 



b. Prism-stereoscope, consisting of one Prism and two Drawings. 

 The stereoscope just described fulfils the condition of the 



complete identity of both projections, and besides this possesses 

 the advantage of a total reflexion, by which incidental images are 

 avoided; but it is applicable to simple reversions only. The 

 following instrument is free from this limitation. In simple 

 reversions, two copies of the same drawing are laid side by side. 

 One of them is viewed through the prism, and the image thus 

 obtained is projected upon the other which is viewed with the 

 naked eye. When the drawings are unsymmetrical, that intended 

 for the right eye is placed right, and viewed through the prism 

 held before the right eye ; the image thus obtained is projected 

 upon the second drawing, which is observed by the naked left 

 eye, and which must be a copy of the looking-glass image of the 

 drawing intended for the left eye in the common stereoscope. 



c. Prism-stereoscope, consisting of a Reversion-prism and two 

 Drawings. 



The reversion-prism reverses an object completely, as well in 

 respect to right and left as to top and bottom. The unsymme- 

 trical projections intended for the common stereoscope are placed 

 in reversed positions beside each other, and the image seen 

 through the reversion- prism is projected upon the other drawing 

 which is observed with the naked eye, 



d. Prism-stereoscope, consisting of two Prisms and two Drawings. 

 Two equal isosceles right-angled prisms are held one before 



