Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER III. DISPERSION. (45) We have hitherto supposed light to be simple or homogeneous. The light of the Sun, however, and most of the lights, natural or artificial, with which we are acquainted, are compound, each ray consisting of an infinite number of rays differing in colour and refrangibility. This important discovery we owe to Newton. We shall briefly describe the principal experiments by which its truth was established. (46) When a beam of solar light is admitted into a darkened room through a small circular aperture, and received on a screen at a distance, a circular image of the Sun will be depicted there, whose diameter will correspond to that of the hole. If now the light be intercepted by a prism, having its refracting edge horizontal and perpendicular to the incident beam, the image of the Sun will be thrown upwards by the refraction of the prism, and will be no longer white and circular, but coloured and oblong; the sides which are perpendicular to the edge of the prism being rectilinear and parallel, and the extremities semicircular. The breadth of this image, or spectrum (as it is called), is equal to the diameter of the unrefracted image of the Sun, but its length is much greater. Now if the solar beam consisted of rays having all the same refrangibility, the refracted image should be circular, and of the same dimensions as the unrefracted image, from which it should differ only in position. For the rays composing the beam, being parallel at their incidence on the prism, must (on this supposition) be equally refracted by it, and therefore continue parallel after refraction. This not being the case, we conclude that the rays composing the incident beam are of different degrees of refrangibility (the more refrangible rays going to form the upper p...