CAMERA OBSCURA, an optical apparatus consisting of a darkened chamber (for which its name is the Latin rendering) into which light is admitted through a convex lens fixed in one end of the chamber. This lens forms an image of external objects on the other end of the camera (fig. r). The end of the camera on which the image is formed, the screen, may be made of translucent paper or ground glass, in which case the image is viewed from outside the camera, or it may be an opaque surface, the camera being sufficiently large to allow the head and shoulders, or even the whole body of the observer to be introduced into the chamber, in which case the image is viewed from inside the cam era. The image formed on the screen is a real or actual image, not virtual as in camera lucida (q.v.). Frequently a sloping mir ror (fig. 2, a) or 9o° prism (fig. 2, b) is added to the simple optical system ; but more generally the mirror and lens are com bined into a single piece of worked glass represented in section in fig. 3. 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 surface b c so as to form an image on the sheet of paper d e. The curved surface takes the place of the lens in fig. 2, a, 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.
Other modifications of the optical system have been made (see PHOTOGRAPHY). The camera obscura has come into use with sub marine vessels, the periscope (q.v.) being simply a camera ob scura under a new name. The camera obscura was extensively used in sketching from nature before the introduction of pho tography.
History.—The invention of this instrument has generally been ascribed to Giovanni Bat tista della Porta, but the prin ciple of the simple camera ob scura, 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, and all he seems really to have done was to popularize it. From the opening passage of Euclid's Op tics (c. 30o B.e.), it would appear that the phenomena of the simple darkened room were used by him to demonstrate the rectilinear propagation of light by the pas sage of sunbeams or the projec tion of the images of objects through small openings in win dows, etc. 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 Aristotle has further noted that 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 projec tion 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 — corre sponds with the passage of light from an object through an aper ture in a darkened place, from which it falls upon a surface fac ing the aperture. Some two hun dred years later, between A.D. 1266 and 1279, these problems were taken up by three almost contemporaneous writers on op tics, two of whom, Roger Bacon (De Speculis and Perspective) and John Peckham (Perspective Communis), were Englishmen, and Vitello or Witelo (Optics), who was known to have. been of Polish extraction.
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 Mura tori'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 demon strations were of two kinds, one nocturnal, showing the moon and bright stars, the other diurnal, for day scenes. This descrip tion 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 fig ured 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.
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 mathematiques en Italie, 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 re versed 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 4o4b 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.
The first published account of the simple camera obscura was discovered by Libri in a translation of the Architecture of Vitru vius, with commentary by Cesare Caesariano, one of the architects of Milan cathedral, published at Como in 1521, shortly after the death of Leonardo, and some twenty years before Porta was born. He describes an experiment made by a Benedictine monk and architect, Dom Papnutio or Panuce, of the same kind as Leonardo's but without the demonstration.
About the same time Francesco Mauro lico, or Maurolycus, the eminent mathe matician of Messina, in his Theoremata de Lumine et Umbra, written in 1521, fully investigated the optical problems con nected with vision and the passage of rays of light through small apertures with and without lenses, and made great advances in this direction over his predecessors. He was the first correctly to solve Aristotle's problem, stated above, and to apply it practically to solar observations in a dark ened room 5) . Eras mus Reinhold has described the method in his edition of G. Purbach's Theoricae Novae Planetarum (1542), and probably got it from Maurolycus. He says it can also be applied to terrestrial objects, though he only used it for the sun. His pupil, Rainer Gemma-Frisius, used it for the observation of the solar eclipse of Jan uary 1544 at Louvain, and fully described the methods he adopted for making measurements and drawings of the eclipsed sun, in his De Radio Astronomico et Geometrico (1545). He says they can be used for observation of the moon and stars and also for longitudes. The same arrangement was used by Copernicus, Tycho Brahe, by M. Moestlin and his pupil Kepler —the latter applying it in 1607 to the observation of a transit of Mercury—also by Johann Fabricius, in 161I, for the first observa tions of sun-spots. It is interesting to note this early employment of the camera obscura in the field of astronomical research, in which its most recent achievements have been of such pre-eminent value.
The addition of optical appliances to the simple dark chamber for the purpose of seeing what was going on outside, was first described by Girolamo Cardan in his De Subtilitate (155o), as noted by Libri. The sun shining, he fixed a round glass speculum (orbem e vitro) in a window-shutter, which being closed, the images of outside objects would be seen transmitted through the aperture on to the opposite wall, or better, a white paper screen suitably placed. The account is not very clear, but seems to imply the use of a concave mirror rather than a lens, which might be suggested by the word orbem. He refers to Maurolycus' work with concave specula.
We now come to Giovanni Battista delta Porta, whose account of the camera obscura in the first edition of the Magia Naturalis, in four books (1558, lib. iv. cap. 2), is very similar to Caesariano's —a darkened room, a pyramidal aperture towards the sun, and a whitened wall or white paper screens, but no lens. He discloses as a great secret the use of a concave speculum in front of the aperture, to collect the rays passing through it, when the images will be seen reversed, but by prolonging them beyond the centre they would be seen larger and unreversed. This is much the same as Cardan's method published eight years earlier, but though more detailed is not very clear. He then notes the application to portraiture and to painting by laying colours on the projected images. Nothing is said about the use of a lens or of solar obser vations. The second edition, in which he in the same words dis closes the use of a convex lens in the aperture as a secret he had intended to keep, was not published till 1589, thirty-one years after the first. In this interval the use of the lens was discovered and clearly described by Daniello Barbaro, a Venetian noble, patriarch of Aquileia, in his work La Pratica delta perspettiva (p. 192), published in 1568, or twenty-one years before Porta's mention of it. The lens used by Barbaro was an ordinary convex or old man's spectacle-glass ; concave, he says, will not do. He shows how the paper must be moved till it is brought into the focus of the lens, the use of a diaphragm to make the image clearer, and also the application of the method for drawing in true perspective. That Barbaro was really the first to apply the lens to the camera obscura is supported by Marius Bettinus in his Apiaria (1645), and by Kaspar Schott in his Magia Uni versalis (1657), the former taunting Porta with the appropria tion.
In an Italian translation of Euclid's Optica, with commentary, Egnacio Danti (1573), after discussing the effects of plane, con vex and concave reflectors, fully describes the method of showing reversed images passing through an aperture in a darkened room, and shows how, by placing a mirror behind the aperture, unre versed images might be obtained, both effects being illustrated by diagrams. F. Risner, who died in 1580, also in his Opticae (16o6) very clearly explained the reversal of the images of the simple camera obscura. He notes the convenience of the method for solar observations and its previous use by some of the ob servers already mentioned, as well as its advantages for easily and accurately copying on an enlarged or reduced scale, especially for chorographical or topographical documents. This is probably the first notice of the application of the camera to cartography and the reproduction of drawings, which is one of its principal uses at the present time. In the Diversarum Speculationum Mathemat icarum et Physicarum (1585), by the Venetian Giovanni Battista Benedetti, there is a letter in which he discusses the simple camera obscura and mentions the improvement some one had made in it by the use of a double convex lens in the aperture ; he also says that the images could be made erect by reflection from any plane mirror.
Thus the use of the camera and of the lens with it was well known before Porta published his second edition of the Magia Naturalis in 1589. In this the description of the camera obscura is in lib. xvii. cap. 6. The use of the convex lens, which is given as a great secret, in place of the concave speculum of the first edition, is not so clearly described as by Barbaro; the addition of the concave speculum is proposed for making the images larger and clearer, and also for making them erect, but no details are given. He describes some entertaining peep-show arrangements, possibly similar to Alberti's, and indicates how the dark chamber with a concave speculum can be used for observing eclipses. There is no mention whatever of a portable box or construction beyond the darkened room, nor is there in his later work, De Refractione Optices Parte in which he discusses the analogy between vision and the simple dark room with an aperture, but incorrectly. Though Porta's merits were undoubtedly great, he did not invent or improve the camera obscura. His only novelty was the use of it as a peep-show ; his descriptions of it are vague, but being published in a book of general reference, which became popular, he acquired credit for the invention.
The first to take up the camera obscura after Porta was Kepler, who used it in the old way for solar observations in 1600, and in his Ad Vitellionem Paralipomena (1604) discusses the early problems of the passages of light through small apertures, and the rationale of the simple dark chamber. He was the first to describe an instrument fitted with a sight and paper screen for observing the diameters of the sun and moon in a dark room. In his later book, Dioptrice (1611), he fully discusses refraction and the use of lenses, showing the action of the double convex lens in the camera obscura, with the principles which regulate its use and the reason of the reversal of the image. He also demonstrates how enlarged images can be produced and projected on paper by using a concave lens at a suitable distance behind the convex, as in modern telephotographic lenses. He was the first to use the term camera obscura, and in a letter from Sir H. Wotton written to Lord Bacon in 162o we learn that Kepler had made himself a portable dark tent fitted with a telescope lens and used for sketching landscapes. Further, he extended the work of Maurolycus, and demonstrated the exact analogy between the eye and the camera and the arrangement by which an inverted image is produced on the retina.
In 1609 the telescope came into use, and the danger of observ ing the sun with it was soon discovered. In 1611 Johann Fabricius published his observations of sun-spots and describes how he and his father fell back upon the old method of projecting the sun's image in a darkened room, finding that they could observe the spots just as well as with the telescope. They do not seem to have used a lens, or thought of using the telescope for projecting an enlarged image on Kepler's principle. This was done in 1612 by Christoph Scheiner, who fully described his method of solar observation in the Rosa Ursina (1630), demonstrating very clearly and practically the advantages and disadvantages of using the camera, without a lens, with a single convex lens, and with a telescopic combination of convex object-glass and concave enlarg ing lens, the last arrangement being mounted with an adjustable screen or tablet on an equatorial stand. Most of the earlier astro nomical work was done in a darkened room, but here we first find the dark chamber constructed of wooden rods covered with cloth or paper, and used separately to screen the observing-tablet.
Various writers on optics in the 17th century discussed the principle of the simple dark chamber alone and with single or compound lenses, among them Jean Tarde (Les Astres de Borbon, 1623); Descartes, the pupil of Kepler (Dioptrique, 1637); Bettinus (Apiaria, 1645) ; A. Kircher (Ars Magna Lucis et Umbrae, 1646) ; J. Hevelius (Selenographia, 1647) ; Schott (Magic Universalis Naturae et Artis, 1674) ; C. F. M. Deschales (Curses, seu Mundus Mathematicus, 1674) ; Z. Traber (Nereus Opticus, 1675), but their accounts are generally more interesting theoretically than as recording progress in the practical use and development of the instrument.
The earliest mention of the camera obscura in England is prob ably in Francis Bacon's De Augmentis Scientiarum, but it is only as an illustration of the projected images showing better on a white screen than on a black one. Sir H. Wotton's letter of 162o was published in 1651 (Reliquiae Wottonianae, p. 141) and in 1658 a description of Kepler's portable tent camera for sketching, taken from it, was published in a work called Grap1iice, or the most excellent Art of Painting, but no mention is made of Kepler. In W. Oughtred's English edition (1633) of the Recreations mathe matiques (162 7) of Jean Leurechon ("Henry van Etten") there is a quaint description, with figures, of the simple dark chamber with aperture, and also of a sort of tent with a lens in it and the projection on an inner wall of the face of a man standing outside. The English translation of Porta's Natural Magick was published in 1658.
Robert Boyle seems to have been the first to construct a box camera with lens for viewing landscapes. It is mentioned in his essay On the Systematic or Cosmical Qualities of Things (ch. vi.), written about 157o, as having been made several years before and since imitated and improved. It could be extended or shortened like a telescope. At one end of it paper was stretched, and at the other a convex lens was fitted in a hole, the image being viewed through an aperture at the top of the box. Robert Hooke, who was some time Boyle's assistant, described (Phil. Trans., 1668, 3, P. a camera lucida on the principle of the magic lantern, in which the images of illuminated and inverted objects were pro jected on any desired scale by means of a broad convex lens through an aperture into a room where they were viewed by the spectators. If the objects could not be inverted, another lens was used for erecting the images. From Hooke's Posthumous Works (1705), p. 127, we find that in one of the Cutlerian lectures on Light delivered in 168o, he illustrated the phenomena of vision by a darkened room, or perspective box, of a peculiar pattern, the back part, with a concave white screen at the end of it, being cylindrical and capable of being moved in and out, while the fore part was conical, a double convex lens being fixed in a hole in front. The image was viewed through a large hole in the side. It was between 4 and 5 f t. long.
Johann Zahn, in his Oculus Artificialis Teledioptricus (1685 1686) described and figured two forms of portable box cameras with lenses. One was a wooden box with a projecting tube in which a combination of a concave with a convex lens was fitted, for throwing an enlarged image upon the focusing screen, which in its proportions and application is very similar to our modern telephotographic objectives. The image was first thrown upon an inclined mirror and then reflected upwards to a paper screen on the top of the box. In an earlier form the image is thrown upon a vertical thin paper screen and viewed through a hole in the back of the camera.
Most of the writers already noticed worked out the problems connected with the projection of images in the camera obscura more by actual practice than by calculation, but William Moly neux, of Dublin, seems to have been the first to treat them mathematically in his Dioptrica Nova (1692), which was also the first work in English on the subject, and is otherwise an interesting book. He has fully discussed the optical theory of the dark cham ber, with and without a lens, and its analogy to the eye. He does not, however, mention the camera obscura as an instrument in use, but in John Harris's Lexicon Technicum (17o4) we find that the camera obscura with the arrangement called the "scioptric ball," and known as scioptricks, was on sale in London, and after this must have been in common use as a sketching instrument or as a show.
Sir Isaac Newton, in his Opticks (17o4), explains the principle of the camera obscura with single convex lens and its analogy with vision in illustration of his seventh axiom, which aptly embodies the correct solution of Aristotle's old problem. He also made great use of the simple dark chamber for his optical experi ments with prisms, etc. Joseph Priestley (1772) mentions the application of the solar microscope, both to the small and portable and the large camera obscura. Many patterns of these two forms for sketching and for viewing surrounding scenes are described in W. J. Gravesande's Essai de perspective (1711), Robert Smith's Compleat System of Optics (1738), Joseph Harris's Treatise on Optics (1775), Charles Hutton's Philosophical and Mathematical Dictionary, and other books on optics and physics of that period. The camera obscura was first applied to photography (q.v.) prob ably about 1794, by Thomas Wedgwood. His experiments with Sir Humphry Davy in endeavouring to fix the images of natural objects as seen in the camera were published in 1802.