CAMERA OBSCURA.
Camera Obscura, a Latin name, meaning literally a dark chamber, belongs to an instrument invented by Baptista Porta in the sixteenth century.
The principle involved in the simplest and most refined forms is the same, and may be illustrated by the following experiment: Let a small hole be cut in an opaque window-shade, and the room darkened. If, now, the beam of light entering the room by this hole be intercepted by a sheet of white paper, held at a small distance from the hole, an inverted image of objects without will be seen upon the paper. By placing a small convex lens over the hole this image is rendered much more distinct. It will also be found, that at a certain distance from the hole the image attains the sharpest or clearest outline, and that if the paper be removed from this point to any position either nearer to the hole or further from it, the image becomes indistinct and confused. At the point of greatest clearness the image is said to be focused. Such being the principle of the camera, it is evident that in practice the instrument may assume many forms, provided always that it consists of a darkened box or chamber, having a hole at one end for the insertion of a lens or combination of lenses, and at the other a screen, generally made of ground glass, on which to receive the image. One of the first home-made cameras I remember seeing was constructed by a boy friend many years ago. In it he used a lens from an old ship’s spy-glass, which still remained incased in its brass tube. Fig. 1 gives a view of this form of camera. As every boy is not as fortunate as my friend in having a brass mounting for his lens, it would be well to inclose it in a small tube of papier-maché or pasteboard, so that it may be moved in or out of the opening at will. The box itself was made of cigar-box wood, with the cover sawed in two parts. After the hole had been cut at one end and the lens inserted, a piece of looking-glass was placed obliquely across the lower corner of the other end of the box, the longer piece of the cover nailed on the front part of the top, and a piece of ground glass carefully fitted, with the ground side downward, over the remaining open space; the smaller part of the cover was then fastened on one side with small pieces of tape. When not in use, this little cover fell down over the glass, but when any object was to be viewed the little lid was lifted into the position in the cut, and served as a shield to the ground glass beneath. A piece of black cloth thrown over this cover, and allowed to fall over the triangular side-openings, so as to still further prevent outside light from reaching the ground glass, is a great improvement.
In the diagram, the dotted lines show the course of the light from the object in view, through the lens (where the rays cross each other) to the looking-glass, and thence to the ground glass above.
A SIMPLER FORM OF THE CAMERA OBSCURA.
A simpler form of the camera obscura is seen in Fig. 2. Here the case is a small soap or spice box, the lens a convex spectacle-glass, and the board marked b a partition, serving as a screen upon which the image is thrown. In this form the lens may be fixed in the end of the box if desired, which is much easier than adjusting it in a sliding tube. The focal distance of an ordinary spectacle-glass averages about twelve or fourteen inches, and the box should be, of course, somewhat longer than the focal length of the glass used. A glass from “near-sighted” spectacles will not do, as it is concave instead of convex.
The box is now pointed out of the window, at some well-marked object, such as a sun-lighted building, and the partition-board moved backward and forward, till the point is found at which the image on it is best defined. Then the board may be fastened (as a, b, c, d, Fig. 2) and the top put on, of which the end should be cut off about four inches from the screen, as shown in the figure.
A piece of black cloth thrown over the head, and completely covering the ends of the opening, renders the image more distinct.
THE SKETCHING CAMERA.
This form of camera may be also used for sketching from nature, by raising it on end, and providing it with an inclined mirror, as shown in Fig. 4. The opening a, b, c, d, should in this case be sufficiently large to admit easy play of the hand in sketching, and also allow an unobstructed view of the image. The mirror may be prepared without much difficulty. Get the glazier to cut for you a piece of looking-glass three or four inches square, and cover the back with a piece of thick paper or card, to prevent scratching; then take a wire of sufficient length, and double it as in A; now bend this double wire in the form of B, letting the ends come at a and b, and placing your mirror, face downward, upon the frame as in C; hold it in place by two bands of strong paper, passed around the glass and wires, and pasted strongly on the back, at the top and bottom of the mirror. When in use this frame is placed over the lens, and reflects the image down upon the drawing-paper placed on the screen below. The four round-headed screws in the top are intended for attaching an opaque curtain to the box, which, however, is only needed when the artist is working in the open air. Then the dark curtain is buttoned in place, and falls over the head and shoulders, completely shielding the image from any invading rays of light which might otherwise confuse the draughtsman.
As a matter of fact, in using the sketching camera, it is necessary to turn one’s back to the objects sketched, if it is desired to draw them in an upright position. I have represented the artist facing the house, as it would look strangely in the illustration to see him seated with his back to the view; but he is compelled, in consequence, to draw his house, sister, and everything else which is included in the image thrown upon his paper upside down, as a penalty for appearances.
THE DARKENED ROOM.
To those boys living in the country, and having a wide, extended landscape stretched out before their windows, the “darkened room” is a very interesting feature. It simply requires a room which can be made perfectly dark. At the window (if there are more than one) commanding the broadest prospect have a perfectly tight, opaque screen fitted, with a small hole cut in the lower part for the insertion of the lens. Over this fasten a small mirror to receive the image, at such an angle as will throw the reflection down upon a stand placed two or three feet from the window, and thus make it possible for the spectator to view the scene in its normal condition. Should you be desirous of having the whole sweep of the horizon at your command in the darkened room, a simply constructed frame-work is necessary for the accommodation of the movable mirror, and also for the lens; this would further necessitate the cutting of a larger hole in the curtain. Fig. 5 represents this arrangement; a being the movable lens, which can be readily taken from its socket if desired; b, the hole in the bottom of the bracket, which should correspond to a larger hole in the shelf d, upon which the bracket rests, and can be easily turned in any direction desired. This hole should be large enough to allow the passage of all the diverging rays, and c, a small mirror, fitted like the one for the sketching camera just described, to receive the image and reflect it down through b upon the stand, or a sheet placed upon the floor for the screen. The height of the shelf d from the floor is determined by the focal length of the lens, and must be decided by experiment before the hole is cut in the shade. It is fastened in place by strings attached to small screw-eyes at its corners, and tied upon tacks driven into the window-frame. The arrangement shown in Fig. 5 can be moved on the shelf, so as to face the lens toward any portion of the view commanded by the window.
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