CAMERA LU'CIDA (Lat., light chamber), A device fitted to the eyepiece of a compound microscope to enable the observer to trace upon a sheet of paper the magnified image of the ob ject as seen. It is eonstrueted in various forms, the simplest of which is a small plate of glass attached to the eyepiece at an angle of 45 de grees. as shown in Fig. 1. A portion of a slide or cover glass fixed to the eyepiece with wax can be employed for this purpose. In sang the camera lueida, the tube of the microscope is placed in a nearly horizontal position, and a sheet of paper laid on the table beneath the eyepiece.
The rays coming through the eyepiece are re Ile•ted into the eye from the glass, though com ing apparently from the paper below, where an enlarged image of the object is seen. The out line of the image can readily be traced with a pencil. and in this way an accurate representa tion of the object can be quickly made. Instead of the simple mirror of glass, Silmmering em ployed a flat, circular piece of polished steel or speculum metal. whose diameter was somewhat smaller than that of the pupil of the eye. The rays are reflected vertically, and enter the eye, as do also those from the paper which come to the eye, after passing by the edges of the mir ror. A prism, so constructed that total reflec tion occurs one or more times, is often employed. and the arrangement of Wollaston, which is fre quently used, is illustrated in the figures and described below. It consists of a small quadri lateral prism of glass, which Fig. 2 shows in perpendicular section, held in a brass frame, which is fitted in front of the eyepiece by a suita ble mounting. or attached to an upright rod. having at its lower end a screw-clamp, to fix it to the edge of a table. The prism, being at the height of shout a foot front the table, has its upper face horizontal. Two of its faces, as in the figure, are at right angles: the eontiguous faces make respectively with them angles of so that the remaining obtuse angle con tains 135°. Rays coining from an object and falling nearly perpendicularly on the first sur face enter the prism, and undergo total reflection at the contiguous surface: they then fall at the same angle on the next surface. and are totally
reflected again: finally, they emerge nearly per pendicular to the remaining surface. An eye, as in the figure, then receives the emergent pencil through one part of the pupil, so that an image of the object is seen projected upon a sheet of paper upon the table. The rays from the paper and pencil passing the edge of the prism enter the other part of the pupil; and the pencil and image being seen together upon the paper, a sketch of the latter can be made. There is. how met., a practical difficulty—the image and the drawing-pencil are at distances sensibly different from the eye, and so cannot he seen together dis tinctly at the same lime. To obviate this, a plate of metal, with a small aperture, as an eye hole, is placed at the edge under the eye, ,so that the rays through the prism, and those from the drawing-peneil, which both pass through the eye-hole. form only very small pencils. A con vex lens is also sometimes employed for this pur pose. The form of camera Nelda devised by Abbe is also used with the microscope. It con sists of two right angle prisms with their by pothenusal faces placed together and a reflecting mirror, as shown in Fig. 3. The separating sur face, ad, is formed by a thin tilm of silver, ex cept a circular space at be. through which the direct rays from the microscope pass. as indi cated by the arrow On To an eye placed at 0, the rays from the paper arc reflected by the mirror .c.zp. and again at the silver film between the prisms, so that the image of the magnified object appears on the paper. This arrangement is so mounted that it can readily he moved to and from the eyepiece. By using a finely dig bled glass scale as an object, we may obtain projected on the paper a magnified image of the divisions, so that the width of the intervening spaces can be measured. A ratio between the scale divis ion, unmagnified and as they appear on the pa per at it distance of 10 ladies from Ilse eye, will give the magnifying power of the microscope. Consult Carpenter, Time Microscope and its Rem lotions, Sth ed., edited by Dallinger (Philade1 phia, 1901).