“A system by which you can see, in their own colors, in the darkness objects outdoors lighted by the sun,” was Porta’s title for the section. He continued:
If anyone wishes to see this effect, all the windows should be closed, and it would be helpful if the cracks were sealed so that no light may enter to ruin the show. Then in one window make a small opening in the form of a cone with the sun at the base and facing the room. Whiten the walls of the room or cover them with white linen or paper. In this way you will see all things outside lighted by the sun, as those walking in the streets, as if their feet were upwards, the right and left of the objects will be reversed and all things will seem interchanged. And the further the screen is from the opening, proportionately the larger the objects will appear; the closer the paper screen or tablet, is drawn to the hole, the smaller the objects will appear.
Porta also had an explanation of the persistence of vision, so far as it was then understood. As an example, he mentioned that after walking in the bright sun it is difficult to discern objects in the darkness, until our eyes become accustomed to the change—and then we can see clearly in the dim light. To see the natural colors, Porta proposed the use of a concave mirror as the screen for the camera images. He then discussed phenomena resulting from the principal focus of the mirror. He tried to use the parallel to show how we see things rightside up instead of upside down. But his knowledge was not sufficient for that purpose, for he held that the seat of vision was at the center of the eye, as the focus of a concave mirror or lens system. In this he was not correct, according to modern experiments, but at least it was a plausible theory.
As a third point in his description of uses of the natural camera Porta said, “Anyone not knowing how to draw can outline the form of any object through the means of a stylus.” Here was Alberti’s camera lucida, or the camera adopted for the use of painters and designers. Porta instructed his readers to learn the colors of the object and then when it was thrown on the screen it would be easy to trace and paint in natural colors. He pointed out another interesting and important fact—a candle or lamp could be used as the light source instead of the sun.
Porta concluded his account of 1558 with an assertion that the system could be used to deceive and to do tricks through the aid of other devices. His last words on the subject were confusing: “Those who have attempted these experiments have produced nothing but trifles, and I do not think it has been invented by anyone else up to now.” Earlier in his account he mentioned that he was now revealing what he thought should be kept a secret.
Roger Bacon, Alberti and Leonardo da Vinci and others were figuratively watching Porta when he wrote those lines and made those experiments. Even the same words about seeing people on the streets outside go back to Bacon, at least; and the use of the camera for drawing to Alberti and Leonardo. It is not clear whether or not Porta actually wished his readers to believe that he had invented the camera obscura which he described or that he had merely found some interesting applications. Perhaps he wanted the whole matter considered a secret.
But though Porta borrowed from the ancients without giving them credit, he deserves praise for publishing descriptions, following tests which he himself must have made. As in all sciences, the prehistory of the motion picture had experimenters and popularizes—and not infrequently the two functions were separated by a considerable period.
The developments claimed by Porta in the second edition of Natural Magic published in 1589 had been described previously by others. Once again he was a copier and popularizer rather than an inventor and discoverer. And that seems proper for a man who was by profession a playwright with a hobby interest in secret things, especially those relating to natural phenomena.
During the three decades prior to 1589, important developments were made in the science of optics. Both Barbaro and Benedetti described camera obscura systems fitted with lenses to improve the images, and E. Danti, an editor and translator, explained in 1573 how an upright, instead of an upside down, image could be shown through the use of a lens-mirror system.
Monsignor Daniello Barbaro published at Venice, in 1568, La Pratica della Perspettiva, “The Practice of Perspective”, a book on optics. He describes the instrument designed by Alberti, the camera lucida, and gives an illustration of it. As in the case of Benedetti, Barbaro’s chief title to memory is that he introduced the projection lens to the natural camera, thereby enlarging its scope. Without any lenses even a modern camera would give only inferior results and motion pictures would not be practical. It is also said Barbaro introduced the diaphragm, which is very important as a means of controlling the light in the camera.