CAMERA LUCIDA. 



231 



seen, and if a piece of white paper be placed there the 

 image will appear to be on it. The prism is generally set in 

 a frame of brass, in the manner exhibited by fig, 99, all 

 parts of it being covered over except the side next the eye- 

 piece and a small portion of the edge to which the eye is to be 

 applied ; the frame is capable of being adapted to either of the 

 eye-pieces by a short tube. The prism itself has two sUght 

 adjustments, one to bring its upper face horizontal, and the 

 other to make the image of the object fall flat on the paper 

 on which it is to be drawn. A lens is generally placed under 

 the camera, in order that the rays of light from the pencil 

 employed in sketching and the object itself may be seen under 

 the same angle. Several contrivances have been had recourse 

 to, in order to simplify certain difficulties that 

 arise in the use of this instrument ; one of these 

 plans is shown in section by fig. 150, and con- 

 sists of a mirror, M, composed of a thin piece 

 of dark coloured glass cemented to a piece of 

 plate glass, inclined at an angle of 45"^, in front 

 of the first lens of the eye-piece, E. The light 

 escaping from the object, through the lens, E, 

 is assisted in its reflection upwards to the eye 

 by the dark glass ; and this, says Mr. Koss,* 

 " effects a fui'ther useful purpose of rendering the paper less 

 briUiant ; and thus enabling the eye better to see the reflected 

 image." If required, a double convex lens, 

 L, may be placed beneath the mirror, as in 

 the case of the prism. The polished steel 

 disc, the invention of Soemmering, before 

 described at page 145, may be employed as a 

 substitute for the camera, over which it is 

 said to have some few advantages. 



Mr. Powell supplies with some of his 

 microscopes an apparatus represented in fig. 

 151 ; it consists of a plate of neutral tint 

 glass fitted in a frame, and so mounted as to 

 be capable of being applied to the eye-piece; 

 Article " Microscope,'' Pennj Cyclopedia. 



Fig. 150. 



