ole.

5. _Variation of spectra in respect to distinctness and size; with a new
way of magnifying objects._

1. It was before observed, that when the two colours viewed together were
opposite to each other, as yellow and blue, red and green, &c. according to
the table of reflections and transmissions of light in Sir Isaac Newton's
Optics, B. II. Fig. 3. the spectra of those colours were of all others the
most brilliant, and best defined; because they were combined of the reverse
spectrum of one colour, and of the direct spectrum of the other. Hence, in
books printed with small types, or in the minute graduation of
thermometers, or of clock-faces, which are to be seen at a distance, if the
letters or figures are coloured with orange, and the ground with indigo; or
the letters with red, and the ground with green; or any other lucid colour
is used for the letters, the spectrum of which is similar to the colour of
the ground; such letters will be seen much more distinctly, and with less
confusion, than in black or white: for as the spectrum of the letter is the
same colour with the ground on which they are seen, the unsteadiness of the
eye in long attending to them will not produce coloured lines by the edges
of the letters, which is the principal cause of their confusion. The beauty
of colours lying in vicinity to each other, whose spectra are thus
reciprocally similar to each colour, is owing to this greater ease that the
eye experiences in beholding them distinctly; and it is probable, in the
organ of hearing, a similar circumstance may constitute the pleasure of
melody. Sir Isaac Newton observes, that gold and indigo were agreeable when
viewed together; and thinks there may be some analogy between the
sensations of light and sound. (Optics, Qu. 14.)

In viewing the spectra of bright objects, as of an area of red silk of half
an inch diameter on white paper, it is easy to magnify it to tenfold its
size: for if, when the spectrum is formed, you still keep your eye fixed on
the silk area, and remove it a few inches further from you, a green circle
is seen round the red silk: for the angle now subtended by the silk is less
than it was when the spectrum was formed, but that of the spectrum
continues the same, and our imagination places them at the same distance.
Thus when you view a spectrum on a sheet of white paper, if you approach
the paper to the eye, you may diminish it to a point; and if the paper is
made to recede fro