The first department in the field which Newton explored with characteristic success was the study of optics. Philosophers were busy with inquiries into the nature of light. It had been long believed that every colored ray is equally refracted when passing through a lens. Newton determined to analyze the prismatic hues. He made a hole in a window-shutter, and darkening the room, let in a portion of light, which he passed through a prism. The _white_ sunbeam formed a circular image on the opposite wall, but the _prismatic colors_ formed an image five times as long as it was broad. He was curious to know how this came to pass. Satisfied that the length of the image in the latter case did not arise from any irregularity in his glass, or from any differences in the incidence of light from different parts of the sun's disk, or from any curvature in the direction of the rays, he concluded, after thorough reflection, that light is not _homogeneous_, but that it consists of rays of diverse refrangibility. The red hue he saw was less refracted than the orange, the orange less refracted than the yellow, and the violet more than any of the rest. These important conclusions he applied in the construction of the first reflecting telescope ever used in the survey of the heavens, and an instrument is preserved in Trinity College Library bearing the inscription, "Invented by Sir Isaac Newton, and made with his own hands, 1671." At the request of the Royal Society, he published in the "Transactions" an account of his optical discoveries, and proved that white light is a compound of seven prismatic colors.