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 thus that the red rays are the least refrangible, and the violet ones the most.

123. Sir Isaac Newton, who may be considered as the Father of the physical part of this Science, distinguishes seven independent primary colours which he calls red, orange, yellow, green, blue, indigo, and violet; and he tells us that the parts of the spectrum occupied by these colours are proportional to the intervals of the diatonic scale in music. He found their degrees of refrangibility in passing out of glass into air to be as the numbers 77, 77, 77, 77, 77, 77, 7977 [sic] and 78; those being the values of the sines of refraction to the common sine of incidence 50. Some substances, however, separate the different coloured rays more widely than others, and the dispersive power of media does not appear to depend at all upon their mean refracting power.