Thus as the thickness of the film is altered different colors preponderate, causing the appearance of rings or bands, according to the nature of the experiment. The dark appearance on the screen corresponding to the thinnest part of the film is probably due to refraction of the ray of light reflected from the second surface, consequent in its passing from a rare into a denser medium, and again from the denser medium into the rare, which refraction Lord Rayleigh considers to effect a retardation equivalent to half a wave length. Lord Rayleigh supported this theory of the formation of Newton's rings by several interesting experiments. A beam of light was intercepted by two of Nicol's prisms, one of which acted as a polarizer and the other as an analyzer of the light, so that no light was able to pass through both on to the screen. Between the two prisms a double refractive lens was now placed, in this case a double concave lens of selenite, when the same series of concentric rings observed with the film of air was obtained on the screen, only much more intense, while a wedge of selenite gave the bands of color in the same order as with the soap bubble. But perhaps the most striking proof of the dependence of the colors upon the thickness of the film was shown by the reflection of a beam of light from a piece of mica composed of twenty-four very attenuated plates overlapping each other. With each layer a marked gradation in color was visible. The remainder of the lecture was devoted to an explanation of the determination of the chromatic relations of the colors of the spectrum. Lord Rayleigh at this point made a rather startling statement that any color can be produced by two other colors. As an example of such a formation, a ray of white light was passed separately through a solution of yellow chromate of potash and an alkaline litmus solution, throwing respectively a yellow and violet-blue color upon the screen. When the ray was made to pass through the two solutions successively, an orange-yellow color was obtained upon the screen, which color Lord Rayleigh asserted to be made up of red and green rays. To prove this, the ray of white light was decomposed by means of a prism, and the decomposed rays passed through the two solutions. The one solution was found to exclude all the yellow and orange rays from the spectrum, while the other excluded all the blue and violet rays, so that when the ray had passed through both solutions, only the red and green rays were left. If, instead of allowing the decomposed ray of light to pass through a slit, and thus obtain definite bands in the spectrum, the ray was passed through a circular hole, the red and green colors overlapped each other on the screen, forming by their combination the identical orange-yellow color obtained with the primary white light. It was then stated that if three definite positions be taken in a spectrum in the red, green, and violet bands respectively, and these positions be