OBLIQUE MIRROR OR PRISM. 



top. Thus placed, the draughtsman can trace the outlines of the 

 picture. 



But in the case here supposed, the axis of the lens being ver- 

 tical, the picture would be that of the firmament. To obtain a 

 picture of any part of the surrounding landscape, a plane mirror, 

 A B, is fixed upon a hinge at B, and is regulated in its position by 

 a handle which descends into the box, so that the draughtsman 

 can give it any desired inclination. The effect of this mirror is 

 indicated in the figure by the rays, which, falling upon it, are 

 reflected downwards to the lens. It will be evident, from what 

 has been already explained in our Tract upon " Optical Images," 

 that when this reflector is properly adjusted, a picture of the 

 landscape before it will be reflected towards the lens L B, and by 

 it projected upon the desk of the draughtsman. 



4. The oblique mirror A B, and the lens L, are sometimes 

 replaced with advantage by a prism, such as that represented in 

 fig. 2. The face, a c, of this prism, at which the rays coming from 

 the landscape enter, being convex, these rays are affected exactly 

 as they would be if they entered the convex 



surface, of a lens ; when they fall upon the plane 

 surface, a 6, of the prism, they will be re- 



fleeted from it, according to what has been f 



explained in "Optical Images" (24); thus ~ 

 reflected, they will fall upon the other side, ; | I / 



c b, of the prism ; this side is ground concave, i ! / 



but its concavity being less than the convexity 1 1 1 



of the side a c, the effect of the two sides * 



upon the rays will be the same as that of a meniscus lens, one 

 side of which has the convexity a c, and the other the concavity 

 b c. In such a lens the convexity prevailing over the concavity, 

 the effect will be that of a convex lens. 



The curvatures of the two sides of the prism are so regulated 

 that its focal length shall correspond with the height of the box. 



5. One of the methods of mounting a camera constructed with 

 such a prism, is shown in fig. 3. The prism is mounted in a case, 

 upon a horizontal axis, and its inclination is regulated by milled 

 heads, like the heads of screws, on the outside ; the case on which 

 it is mounted has an opening through which the rays proceeding 

 from the landscape are admitted ; and it can be turned round its 

 vertical axis, so that the opening can be presented in any direc- 

 tion to the surrounding landscape. The apparatus is supported by 

 a triangle, and the draughtsman is surrounded by a curtain, 

 forming a tent, from which the light is sufficiently excluded. The 

 height of the tent, relatively to the table, is of course regulated 

 according to the focal length of the prism. 



205 



cf. 



c*TT^i 



