Some time after this we learn that a German named Schultze made copies of drawings with a mixture of chalk and silver nitrate spread on a level surface. The time of this is doubtful, but it was probably about the year 1700. He passed the light, as he says, through translucent paper (made translucent with oil or wax), and objects placed upon the paper left a white impression on the mixture of chalk and silver nitrate--or, as he called it, “lunar caustic.” This was in about 1700, as I said. About fifty years after this time (and indeed it was a little more, it was seventy years, in 1777) Scheele, the Swedish apothecary’s assistant, took up the examination of this horn silver. It seemed to him well worthy of study; and as the result of his work he obtained the first germs that led to the art of photography. But before Scheele could have prosecuted his researches, and before photography could make any important advances, there were two other discoveries in science--and in optics particularly--that had to be made. The first of these was the decomposition of white light, by Sir Isaac Newton, by which he obtained the prismatic colors; that is to say, the colors that we know as violet, indigo, blue, green, and so on down to the red. That was the first step. The next step was the discovery by Baptiste Porter, an Italian, in Naples, which preceded the discovery of Newton (it was about 1590), that a small opening in a dark chamber produced an inverted image on the wall of the chamber. So that between 1590 and 1666 Baptiste Porter and Sir Isaac Newton paved the way for the researches of Scheele upon the action of light upon this simple substance, as they called it, “luna cornea” or chloride of silver. Now Scheele, therefore, at his time, 1777, knew of the discovery of the prismatic colors, or the decomposition of white light by Sir Isaac Newton, and he made the experiment of submitting this horn silver or silver chloride to the action of light after the light had been passed through a prism and he found the light as we know it to consist of violet, indigo, blue, green, yellow, orange and red. Placing the silver chloride in this band of colors, he discovered the important fact that in the red rays the silver chloride received no change--that there was no change made in it. But, as he got along toward the other end of the spectrum, and got into the green and the blue and the indigo and the violet, he found that the color of the silver chloride changed much more rapidly, and he found that the most active in its effect upon the silver chloride were the blue and violet rays. In addition to this fact he found that the light discolored the silver chloride. Scheele still further proved that the silver chloride was decomposed by the light, and that chlorine gas, or, as he called it, dephlogisticated marine acid gas, was produced. He became acquainted with this previously from his experiments on the mineral braunstein with muriatic acid.