31. John Baptista Porta, of Naples, was contemporary with Maurolicus. He invented the camera obscura, and his experiments with this instrument convinced him, that light was a substance, and that its reception into the eye produced vision. These discoveries corresponded very nearly with those by Maurolicus, although neither of these philosophers had any knowledge of what the other had done. The importance of Porta's discoveries will be evident, when it is observed, that, before his time, vision was supposed to be dependent on what were termed _visual rays_, proceeding from the eye. 32. The telescope was invented towards the latter end of the sixteenth century. Of this, as of many other valuable inventions, accident furnished the first hint. It is said, that the children of Zacharias Jansen, a spectacle-maker, of Middleburg in Holland, while playing with spectacle-glasses in their father's shop, perceived that, when the glasses were held at a certain distance from each other, the dial of the clock appeared greatly magnified, but in an inverted position. 33. This incident suggested to their father the idea of adjusting two of these glasses on a board, so as to move them at pleasure. Two such glasses inclosed in a tube completed the invention of the simplest kind of the refracting telescope. Galileo greatly improved the telescope, and constructed one that magnified thirty-three times, and with this he made the astronomical discoveries which have immortalized his name. 34. John Kepler, a great mathematician and astronomer, who was born at Weir, in Wurtemburg, in the year 1571, paid great attention to the phenomena of light and vision. He was the first who demonstrated that the degree of refraction suffered by light in passing through lenses, corresponds with the diameter of the circle of which the concavity or convexity is the portion of an arch. He very successfully pursued the discoveries of Maurolicus and Porta, and asserted that the images of external objects were formed upon the optic nerve by the concentration of rays which proceed from them. 35. In 1625, the curious discovery of Scheiner was published, at Rome, which placed beyond doubt the fact, that vision depends upon the formation of the image of objects upon the retina. The fact was demonstrated by cutting away, at the back part, the two outside coats of the eye of an animal, and by presenting different objects before it. The images were distinctly seen painted on the naked retina.