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Vitruvius, with commentary by Cesare Caesariano, one of the architects of Milan cathedral, published at Conio 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 Maurolico, or Maurolycus, the eminent mathematician of Messina, in his Theoremata de Lumine et Umbra, written in 1521, fully investigated the optical problems connected 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 darkened room (Cosmographia, 1535). Erasmus 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 January 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 1611, for the first observations of sun-spots. It is interesting to note this early employment of the camera obscura in the field of astronomical research, in which its latest 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 (1550), as noted by Libri. The sun shining, he fixed a round glass speculum (orbem e vitro) in a window-shutter, and then closing it 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 della 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 observations. The second edition, in which he in the same words discloses 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 della 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 Universalis (1657), the former taunting Porta with the appropriation.

In an Italian translation of Euclid’s Optica, with commentary, Egnacio Danti (1573), after discussing the effects of plane, convex 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, unreversed images might be obtained, both effects being illustrated by diagrams. F. Risner, who died in 1580, also in his Opticae (1606) 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 observers 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 Mathematicarum el 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 (1593), 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 1620 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 observing 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 imase on Kepler’s principle. This