Sir Isaac Newton's name would have been immortalized by this discovery alone, even if he had not possessed that transcendent ability which raised him above all other mathematicians and physicists. It is at the same time interesting to know that the ancient author Claudian (A.D. 420) inquires "whether colour really belongs to the substances themselves, or whether by the reflection of light they cheat the eye--_enquires sitve color proprius rerum, lucisne repulsa eludant aciem_." Sir Isaac Newton determined that the spectrum could be divided into 360 equal parts, of which red occupied 45, orange 27, yellow 48, green 60, blue 60, indigo 40, violet 80. He also discovered that if the highly refracted rays, the seven colours, or spectrum were received into [Page 310] a concave mirror or a double-convex lens, that they again united and formed white light. In order to demonstrate the properties of the prism in various positions, the next diagram may be adduced. (Fig. 302.) [Illustration: Fig. 302. A. The ray of light passing through two prisms B placed base to base. In this position the light passes through to the second prism, C, without alteration. At C the decomposition of light occurs, and the spectrum is shown at D D. The top prism at B used singly would reflect the ray to E without decomposing it into the coloured rays.] The rainbow is the most beautiful natural optical phenomenon with which we are acquainted; it is only seen in rainy weather when the sun illuminates the falling rain, and the spectator has the sun at his back. There are frequently two bows seen, the interior and exterior bow, or the primary and secondary, and even within the primary rainbow, and in contact with it, and outside the secondary one, there have been seen other bows beyond the number stated. The primary or inner rainbow consists of seven different coloured bows, and is usually the brightest, being formed by the rays of light falling on the upper parts of the drops of rain. The exterior bow is formed by the rays of light falling on the lower parts of the drops of rain; and in both cases the rays of light undergo refraction and reflection, hence the opinion of Aristotle, that the rainbow is caused only by the reflection of light, is not correct. The first refraction occurs when the rays of light enter, and the second when they emerge from the spheroids of water in the first bow; the refracted rays undergo only one reflection, whereas in the second the brilliancy of the colours is impaired by two reflections. The spectrum from the electric light is one of the most gorgeous exhibitions of colour that can be conceived; and the instruments required for the purpose are illustrated in No. 1 (Fig. 303), whilst the [Page 311] synthesis of the coloured rays and production of white light is shown at No. 2 of the same figure. (Fig. 303.)