792 PROFESSOR PIAZZI SMYTH ON COLOUR, 
pictorial pigment , so far as I know, which, bemg added to blue, can increase 
it in the scale of refrangibility. 
Hence we fall back with all the more respect on NEwrton’s earliest state- 
ment of colour in prism-scopy, as well representing a grand physical action in 
Nature of the most admirable, and otherwise inimitable, kind; when he laid 
down that, ‘every spectral colour has a certain refrangibility, and every degree 
of refrangibility a certain colour, peculiar, constant and unalterable.” 
Sir Davip BREwsTER unfortunately imagined at one time, that he had upset 
that law, by having proved, with a mixture of coloured glasses superposed on 
prisms, that each of the then recognised seven colours of the spectrum existed 
more or less in every part of, and all along, the whole of the spectrum. 
That total mistake (arising chiefly from the impurity of the colours of 
coloured-glasses), has been abundantly disproved by many persons since then ; 
and it was indeed a necessary obstruction to be removed, before the modern 
spectrum analysis of MM. Bunsen and Kircnorr, could be fairly established. 
But on the other hand, NEwron’s older law which had so much truth in it,— 
and for approximate purposes may be spoken of as true still, is nevertheless, I 
fear, by no means absolutely correct for minute, but measurable, quantities. 
This conclusion arises first, from the peculiar /ocomotive character of colour 
bands in our absorption spectra described on pp. 782, 783: and it was proved 
again still more signally to me last summer when spectroscoping the high sun 
at noonday in Lisbon. For when the instrumental arrangements were pecu- 
liarly such that the solar spectrum colours (made pure by using the narrowest 
possible slit, and mo coloured glass shades and yet eliminating side reflections), 
were so brilliant and decided that no one with tolerable eyes could refuse to 
see, not a mere three, but at deast 15 different and distinct kinds of colour in a 
continued order along the chromatic scale (then 30 feet long, from red to laven- 
der),—it was forced on my attention that any alteration of the brightness of the 
spectrum, caused either accidentally or purposely, would often shift the very 
colours themselves, by name, or by eye-appreciation as such, through one or two 
of the 15 colour-spaces, backwards or forwards as the case might be. At such 
times too, there was no alteration of refrangibility whatever ; either according 
to instrumental construction, or actually observed steadiness of the Fraunhofer 
lines in the field of the measuring telescope: but there was undoubtedly a sort 
of small-range movement of the colour-scale as a whole, though happily only 
through the limits of a small epicycle as it were of adjustment attached to it. 
Hence colours, even of the spectrum, however beautiful and though practi- 
cally pure, are not amenable with full accuracy to the simple Jaw of refraction 
of rays of light in a prism ; and contain some unexplained phenomena still.* 
* This section has been written in February 1879. 
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