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prepossessed with that idea. Most indiv iduals, probably,seeing the spectrum for the first time, would rather dis.tinguish it into three principal tints, red, yellow, andblue, with intermediate shades. To return, however, toNewton: the first question as to the cause which sug-gested itself was, whether the difference of colour mightnot be owing to the difference of thickness of the glassthrough which the different rays had to pass. This wasaccordingly put to the test, by causing two rays to pass,one through the thick part of the prism, another through apart near the edge; but each ray produced a completespectrum of its own : this, therefore, could not be thereal cause. The next suggestion was, that the effectmight be due to irregularities in the glass ; but thismust take place equally in all positions of the prism,and, in fact, be doubled, if the light passed through twoprisms ; accordingly, two prisms of exactly the sameangle were placed together, one being inverted, so thatthey together formed a solid with parallel surfaces, thelight passing through both, came out unaltered, un-coloured, and giving a perfect undistorted image of thesun: this supposition was, therefore, rejected. But therays from the different parts of the sun’s disk form asmall angle with each other, and their incidence on theprism is, therefore, slightly different; might not this bemagnified in the course of two refractions, and so accountfor the effect? This was simply matter of calculation andmeasurement, and was shown to be utterly insufficient.
Newton next enquired whether the rays might not bebent into curve lines, after passing through the prism,and so, in proportion to the degree of inflection, mightfall on different parts of the screen, with different degreesof obliquity. This was refuted by measuring the lengthof the spectrum at different distances from the prism :it was always in exact proportion to the distance, andthe rays consequently strictly rectilinear.
Having disposed of all these suppositions, the questionwas reduced to narrower limits; a perfect exemplificationof the process recommended by Bacon . Newton nowt 4