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.