camera obscura, which was extensively used in sketching from nature before the introduction of photography, although it is now scarcely to be seen except as an interesting side-show at places of popular resort. The image formed on the paper may be traced out by a pencil, and it will be noticed that in this case the image is real—not virtual as in the case of the camera lucida. Generally the mirror and lens are combined into a single piece of worked glass represented in section in fig. 2.
Rays from external objects are first refracted at the convex surface a b, then totally reflected at the plane surface a c, and finally refracted at the concave surface b c (fig. 2) so as to form an image on the sheet of paper d e. The curved surfaces take the place of the lens in fig. 1, and the plane surface performs the function of the mirror. The prism a b c is fixed at the top of a small tent furnished with opaque curtains so as to prevent the diffused daylight from overpowering the image on the paper, and in the darkened tent the images of external objects are seen very distinctly.
Quite recently, the camera obscura has come into use with submarine vessels, the periscope being simply a camera obscura under a new name. (C. J. J.)
History.—The invention of this instrument has generally been ascribed, as in the ninth edition of this work, to the famous Neapolitan savant of the 16th century, Giovanni Battista della Porta, but as a matter of fact the principle of the simple camera obscura, or darkened chamber with a small aperture in a window or shutter, was well known and in practical use for observing eclipses long before his time. He was anticipated in the improvements he claimed to have made in it, and all he seems really to have done was to popularize it. The increasing importance of the camera obscura as a photographic instrument makes it desirable to bring together what is known of its early history, which is far more extensive than is usually recognized. In southern climes, where during the summer heat it is usual to close the rooms from the glare of the sunshine outside, we may often see depicted on the walls vivid inverted images of outside objects formed by the light reflected from them passing through chinks or small apertures in doors or window-shutters. From the opening passage of Euclid’s Optics (c. 300 B.C.), which formed the foundation for some of the earlier middle age treatises on geometrical perspective, it would appear that the above phenomena of the simple darkened room were used by him to demonstrate the rectilinear propagation of light by the passage of sunbeams or the projection of the images of objects through small openings in windows, &c. In the book known as Aristotle’s Problems (sect. xv. cap. 5) we find the correlated problem of the image of the sun passing through a quadrilateral aperture always appearing round, and he further notes the lunated image of the eclipsed sun projected in the same way through the interstices of foliage or lattice-work.
There are, however, very few allusions to these phenomena in the later classical Greek and Roman writers, and we find the first scientific investigation of them in the great optical treatise of the Arabian philosopher Alhazen (q.v.), who died at Cairo in A.D. 1038. He seems to have been well acquainted with the projection of images of objects through small apertures, and to have been the first to show that the arrival of the image of an object at the concave surface of the common nerve—or the retina—corresponds with the passage of light from an object through an aperture in a darkened place, from which it falls upon a surface facing the aperture. He also had some knowledge of the properties of concave and convex lenses and mirrors in forming images. Some two hundred years later, between A.D. 1266 and 1279, these problems were taken up by three almost contemporaneous writers on optics, two of whom, Roger Bacon and John Peckham, were Englishmen, and Vitello or Witelo, a Pole.
That Roger Bacon was acquainted with the principle of the camera obscura is shown by his attempt at solving Aristotle’s problem stated above, in the treatise De Speculis, and also from his references to Alhazen’s experiments of the same kind, but although Dr John Freind, in his History of Physick, has given him the credit of the invention on the strength of a passage in the Perspectiva, there is nothing to show that he constructed any instrument of the kind. His arrangement of concave and plane mirrors, by which the realistic images of objects inside the house or in the street could be rendered visible though intangible, there alluded to, may apply to a camera on Cardan’s principle or to a method of aerial projection by means of concave mirrors, which Bacon was quite familiar with, and indeed was known long before his time. On the strength of similar arrangements of lenses and mirrors the invention of the camera obscura has also been claimed for Leonard Digges, the author of Pantometria (1571), who is said to have constructed a telescope from information given in a book of Bacon’s experiments.
Archbishop Peckham, or Pisanus, in his Perspectiva Communis (1279), and Vitello, in his Optics (1270), also attempted the solution of Aristotle’s problem, but unsuccessfully. Vitello’s work is to a very great extent based upon Alhazen and some of the earlier writers, and was first published in 1535. A later edition was published, together with a translation of Alhazen, by F. Risner in 1572.
The first practical step towards the development of the camera obscura seems to have been made by the famous painter and architect, Leon Battista Alberti, in 1437, contemporaneously with the invention of printing. It is not clear, however, whether his invention was a camera obscura or a show box, but in a fragment of an anonymous biography of him, published in Muratori’s Rerum Italicarum Scriptores (xxv. 296), quoted by Vasari, it is stated that he produced wonderfully painted pictures, which were exhibited by him in some sort of small closed box through a very small aperture, with great verisimilitude. These demonstrations were of two kinds, one nocturnal, showing the moon and bright stars, the other diurnal, for day scenes. This description seems to refer to an arrangement of a transparent painting illuminated either from the back or the front and the image projected through a hole on to a white screen in a darkened room, as described by Porta (Mag. Nat. xvii. cap. 7) and figured by A. Kircher (Ars Magna Lucis et Umbrae), who notes elsewhere that Porta had taken some arrangement of projecting images from an Albertus, whom he distinguished from Albertus Magnus, and who was probably L. B. Alberti, to whom Porta also refers, but not in this connexion.
G. B. I. T. Libri-Carucci dalla Sommaja (1803–1869), in his account of the invention of the camera obscura in Italy (Histoire des sciences mathématiques en Italic, iv. 303), makes no mention of Alberti, but draws attention to an unpublished MS. of Leonardo da Vinci, which was first noticed by Venturi in 1797, and has since been published in facsimile in vol. ii. of J. G. F. Ravaisson-Mollien’s reproductions of the MSS. in the Institut de France at Paris (MS. D, fol. 8 recto). After discussing the structure of the eye he gives an experiment in which the appearance of the reversed images of outside objects on a piece of paper held in front of a small hole in a darkened room, with their forms and colours, is quite clearly described and explained with a diagram, as an illustration of the phenomena of vision. Another similar passage is quoted by Richter from folio 404b of the reproduction of the Codice Atlantico, in Milan, published by the Italian government. These are probably the earliest distinct accounts of the natural phenomena of the camera obscura, but remained unpublished for some three centuries. Leonardo also discussed the old Aristotelian problem of the rotundity of the sun’s image after passing through an angular aperture, but not so successfully as Maurolycus. He has also given methods of measuring the sun’s distance by means of images thrown on screens through small apertures. He was well acquainted with the use of magnifying glasses and suggested a kind of telescope for viewing the moon, but does not seem to have thought of applying a lens to the camera.
The first published account of the simple camera obscura was discovered by Libri in a translation of the Architecture of