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| Fig 2 Newton's sketch of his prism arrangement for Experiment 44 which incorporated the earlier Experiment 4. |
Newton's OptiksIn the 17th Century Newton performed a series of experiments with a single beam of white light and a triangular glass prism. He observed that when the beam of light was shone through a prism, it split into different colours. At the time Newton's theory described light as being made up of particles, or corpuscles as he named them. Newton proposed that the larger the size of the corpuscle the smaller the refractive index.
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| Fig 2 Newton's sketch of his prism arrangement for Experiment 44 which incorporated the earlier Experiment 4. |
In Optiks, Book 1:Part 1 Newton details his 4th Experiment. He made a small hole in the window shutter of a dark chamber which admitted a narrow beam of sunlight. He placed a prism several feet away from the hole and allowed the dispersed beam to shine upon a piece of paper across the room. Figure 2 above shows the original drawing made by Newton of his 44th trial which incorporated the first prism of the 4th Experiment and added a second inverted prism 4.
He writes " In this situation of the prism, viewing through it the said hole, I observed the length of its refracted image to be many times greater than its breadth, and that the most refracted part thereof appeared Violet. The least refracted Red, the middle parts Blue, Green and Yellow in order."8 As he described the Red corpuscles to be of the largest size, this would 'explain' why the Red light appeared at the bottom of the dispersed beam and was bent at a lesser angle. The website Newton's Colours presents arguments regarding Newton's experimental methods and conclusions, one of the major contentions being the limited credibility of his process. Newton's theory of corpuscles is criticized because it is based almost entirely on his 44th trial alone. He found that when he isolated a beam of light refracted through one prism and directed it through a second, the beam remained otherwise unaltered. For Newton this was proof that colours are not modifications of white light but are in fact the original components of white light. He claimed that colored light was pure and could not be separated. 9
Robert Hooke repeated Newton's two-prism experiment but disagreed with his conclusion. Colours, he claimed, came from the prism itself. He also noted that colours could be produced in other experiments not involving refraction. His conclusion indicated that colours had other sources besides white light. Around 1790 a group of university students noted that when they refracted a Violet beam of light through one prism tinges of Red were visible, and if it were directed through two prisms both Red and Yellow tinges were visible. Their conclusion was that colours are inconstant and mutable, disproving Newtons' premise that colored light was light at its purest. In reaction to this Newton claimed that others had used poor quality prisms. Newton had (he said) used good quality flint glass containing lead oxide, free of veins, bubbles and any subtle color. He also added that they used concave prisms while he used convex.
Although Newton was correct in assuming this about the prisms, controversy arises because he is claiming such self-evident results from such extraordinary conditions.9 Newton, and those who support his theory claim it to be so simple and general, but according to his annotations about the specifications of the prism, obtaining the same results as Newton involved a specific, unique criterion. In stating these conditions Newton's results can be seen as non-universal. After the publication of Optiks, Helmholtz, who preferred the spectra produced by diffraction gratings, dismissed Newtons color choice of ROYGBIV remarking " These division are more or less capricious and largely the result of a mere love of calling things by names"4. It can be seen that in the previous quote from his book Optiks, in Experiment 4 Newton first noticed 5 distinct colours, Red, Yellow, Green, Blue and Violet. These five colours agree with the prominent natural bands that can be seen in a rainbow, an example of this is shown in figure 1 above.