Page:The New International Encyclopædia 1st ed. v. 04.djvu/109

* CAMERA LUCIDA. 83 the figure, are at right angles; the contiguous faces make respectively «ith them angles of 6714°, so that the remaining obtuse angle con- tains 135°. Rays coming from an object and falling nearly perpendicularly on the first sur- face enter the prism, and undergo total refiection at the contiguous surface ; they then fall at the same angle on the next surface, and are totally rellected 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- ever, a practical difliculty — the image and the drawing-pencil are at distances sensibly dift'erent from the eye, and so cannot be seen together dis- tinctly at the same time. To obviate this, a plate of metal, with a small aperture, as an eye- hole, is placed at the edge under the eye, ;;o that the rays through the prism, and those from the drawing-pencil, 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 lucida 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 O CAMERA OBSCTJRA. lations, 8th ed., edited by Dallinger (Philadel- phia, 1901). CAMERA OBSCU'RA (Lat., dark chamber). A liglit-tight box, with a convex lens atone end and a screen at the other, on which an image is produced. This screen is generally a piece of W FIO. 3. ABBE'S CAMERA LUCIDA. mirror, as shown in Fig. 3. The separating sur- face, ad, is formed by a thin film of silver, ex- cept a circular space at be, through which the direct rays from the microscope pass, as indi- cated by the arrow Oil. To an eye placed at O, the rays from the paper are reflected by the mirror Sp, 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 numnted that it can readily be moved to and from the eyepiece. By using a finely divided glass scale as an object, we may obtain ])rojected on the paper a magnified inuige 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 appeal- on the pa- per at a distanci! of 10 inches from the eye, will give the magnifying power of the microscope. Consult Carpent<;r, The Microscope and its Heve- FIG. 1. CAMERA OBSCCRA. ground glass, or translucent paper, so that the image may be viewed from behind. The rays of light coming from an object pass through the lens, and when they reach the screen form an image which can be received on a sensitive plate and preserved in the form of a negative. If the screen is placed at the approj^riate focal distance, a sharp representation of an object can be obtained. The himian eye is a simple form of camera obscura, the crystalline lens and retina occupying the relative positions of lens and screen. The lens is not essential for the forma- tion of an image, as with a small opening a picture is also produced. Such an image, while free from distortion, is not apt to be sharp, and a long exposure in making a negative is re- quired. The invention of the camera obscura is of importance on account of its being the prototype of the modern jihotographie camera, and is claimed for a number of celebrated men. The first of these is Roger Bacon, who lived in the Thirteenth Century, and after him comes Albcrti, about two centuries later. Leonardo da Vinci, the famous painter and scientist, about the be- ginning of the Sixteenth Century, says that if you place yourself in an hermetically closed room, facing a building, landscape, or anj' other object directly lighted by the sun, and then cut