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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^[1]. He found their degrees of refrangibility in passing out of glass into air to be as the numbers 77, 771/8, 771/5, 771/3, 771/2, 772/3, 797/8 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^[2].

↑ Dr. Wollaston has determined the division of the spectrum, with great accuracy, by looking through a prism at a narrow line of light; his result is that there are four primary colours: red, green, blue, and violet, occupying, respectively, 16, 23, 36 and 25 parts, in length, of the spectrum.
⁠The blue light at the bottom of the flame of a candle, is easily resolved into five distinct parcels of the following colours, red, light green, dark green, blue, and violet; that of a spirit lamp, which appears quite blue, consists chiefly of green and violet rays.
↑ The difference of the extreme values of the fraction sin inc./sin refr. for glass, appears, according to Newton, to be 1/50. This might be taken as a measure of the dispersive power of the substance, but it is usual to adopt a different expression.
⁠When the angles of incidence and refraction are very small, they are nearly proportional to their sines. If therefore we take a constant small angle $\theta$ for the angle of refraction, the angle of incidence will be $m\theta$ , and will differ according to the value of $m$ . The difference between these two, or ${\overline {m-1\theta }}$ , is the refraction; and if $m_{r}$ and $m_{v}$ be values of $m$ for red and violet rays, the difference of the refractions or the dispersion will be ${\overline {m_{v}-1\theta }}-{\overline {m_{r}-1\theta }}$ or $(m_{v}-m_{r})\theta$ . Its ratio to the refraction will consequently be ${\frac {m_{v}-m_{r}}{m}}$ , taking the mean value for $m$ : this is the usual measure of the dispersive power.
⁠In flint-glass its value is about 0.05; in crown-glass 0.033.

[1] Dr. Wollaston has determined the division of the spectrum, with great accuracy, by looking through a prism at a narrow line of light; his result is that there are four primary colours: red, green, blue, and violet, occupying, respectively, 16, 23, 36 and 25 parts, in length, of the spectrum.
⁠The blue light at the bottom of the flame of a candle, is easily resolved into five distinct parcels of the following colours, red, light green, dark green, blue, and violet; that of a spirit lamp, which appears quite blue, consists chiefly of green and violet rays.

[2] The difference of the extreme values of the fraction sin inc./sin refr. for glass, appears, according to Newton, to be 1/50. This might be taken as a measure of the dispersive power of the substance, but it is usual to adopt a different expression.
⁠When the angles of incidence and refraction are very small, they are nearly proportional to their sines. If therefore we take a constant small angle $\theta$ for the angle of refraction, the angle of incidence will be $m\theta$ , and will differ according to the value of $m$ . The difference between these two, or ${\overline {m-1\theta }}$ , is the refraction; and if $m_{r}$ and $m_{v}$ be values of $m$ for red and violet rays, the difference of the refractions or the dispersion will be ${\overline {m_{v}-1\theta }}-{\overline {m_{r}-1\theta }}$ or $(m_{v}-m_{r})\theta$ . Its ratio to the refraction will consequently be ${\frac {m_{v}-m_{r}}{m}}$ , taking the mean value for $m$ : this is the usual measure of the dispersive power.
⁠In flint-glass its value is about 0.05; in crown-glass 0.033.

[1]

[2]