The combined effect of the two is to produce a spectrum sloping up from left to right. 3) refracts upwards, while the prism Q, which has its refracting edge perpendicular to that of P, refracts towards the right. Newton also made use of the method of crossed prisms, which has been found of great use in studying dispersion. This confirmed his previous conclusion that the rays increase in refrangibility from red to violet. Operating on this beam with a second prism, he found that the homogeneous light was not dispersed, and also that it was more refracted the nearer the point from which it was taken approached to the violet end of the spectrum RV. Some of this light also passed through the second hole, and thus he obtained a narrow beam of practically homogeneous light in a fixed direction (the line joining the apertures in the two screens). By slightly turning the prism P, the position of the spectrum on the first screen could be shifted sufficiently to cause light of any desired colour to pass through. In an experiment similar to that here represented, Newton made a small hole in the screen and another small hole in a second screen placed behind the first. If now the prism P be interposed as in the figure, the whole beam is not only refracted upward, but also spread out into the spectrum RV, the horizontal breadth of the band of colours being the same as that of the original image S. 2) represent a :small hole in the shutter of a darkened room, and OS a narrow beam of sunlight which is allowed to fall on a white screen so as to form an image of the sun at S. This method was employed by Sir Isaac Newton, whose experiments constitute the earliest systematic investigation of the phenomenon. As the light is twice refracted, the dispersion is increased, and the rays, after transmission through the prism, form a divergent system, which may be allowed to fall on a sheet of white paper, forming the wellknown solar spectrum. The simplest way of showing dispersion is to refract a narrow beam of sunlight through a prism of glass or prismatic vessel =containing water or other clear liquid. Dispersion is therefore due to the fact that rays of different colours possess different refrangibilities. The order of the successive colours in all colourless transparent media is red, orange, yellow, green, blue, indigo and violet. ![]() 1) enters obliquely A into the surface of a block of glass at 0, it gives rise to the divergent system of rays ORV, varying continuously in colour from red to violet, the red ray OR being least refracted and the violet ray OV most so. which does not consist of simple vibrations of a definite wave-length, undergoes refraction at the surface of any transparent medium, the different colours corre sponding to the different wave-lengths become separated or. When a beam of light which is not homogeneous in character, i.e.
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