page 780 note * This neat little contrivance, admirable up to a certain extent, was yet found to have a weakness in certain parts of the Spectrum, the nature, and subsequent complete cure of which will be found described in the “Postscript” to this paper, at pp. 802–804.

page 781 note * M. Angstrom's grand Normal Solar Spectrum Map is a type of diffraction projections. “Wavelengths” form there a scale of equal parts along the whole of its length of 137 inches nearly. For one purpose of theory that wave-length scale may be convenient; but evidently Nature declines to be bound by it in other respects; for in the first 30 inches of it, there are only 98 solar lines; and in the last 30, so many as 522; so that they hardly have standing room.

An ingenious idea was proposed some years ago by Prof. Alexander S. Herschel, for employing both a nature-founded scale, and in equal parts on the paper for prismatic spectra almost as observed; or with all their exaggerated length of the violet end. The method consisted in employing, not wave-numbers per inch, but their squares; and it is nearly true for the average of prisms. But its numbers are impracticably large, and it does too little justice to diffraction spectra.

page 782 note * Further, we place the red-end, or natural beginning, of the spectrum to the left-hand because, although a recent reviewer of Mr Kand Capron's “Photographed Spectra” chides him for so doing; and says, “have not Kirchoff, and Huggins placed the red-end to the right;”—yet the opposite plan has been adopted by Fraunhofer, Brewster, Gladstone, Bunsen, Janssen, Eoscoe, Watts, De Willengen per Secchi, Plucker, Hittorf, De Boisbaudran, and lastly Eussell of Sydney in the “Observatory” for March 1879. While we may also claim the practice of the whole European family of nations in writing from left to right, never from right to left.—(Added March 8, 1879.)

page 783 note * See p. 50, par. 3, of “The Spectroscope and its Work, 1877; by Kichard A. Proctor, B.A., Cambridge; being one of the “Manuals of Elementary Science,” prepared under the auspices of “The Committee of General Literature and Education, appointed by the Society for Promoting Christian Knowledge.”

page 784 note * This fluid, as being one of the aniline series of coal-tar dyes, must be of too modern discovery or manufacture to have come under the piercing examination of the late Sir David Brewster; though he had found other coloured solutions having in different, but lesser, degrees the property just mentioned; viz., that with increased thicknesses they transmitted not bluish, but red light only.

page 789 note * See forward to pp. 791 and 792.

page 792 note * This section has been written in February 1879.

page 793 note * The Oxalate of Chromium and Potash was well examined by Sir David Brewster in his best days; and he distinctly observed that though bluish in weak solutions, it allowed only red light to pass through its thicker solutions. Our recently observed effect depends upon that, but is accompanied by something further.

page 795 note * One of Judson's ¼ ounce bottles of concentrated dye, mixed in 20 ounces of hot water and allowed to cool, gives nearly the requisite strength and darkness, in a flat bottle one inch thick internally. On trying solutions of various strength in an angular or prism-shaped bottle the dispersion was not found to be anomalous as with fuchsine, but normal as with glass, though with the addition of looking through the glass-prism with a flat film of Judson's green interposed, and thereby reducing the whole continuous spectrum of daylight to a red band and a green band, with a black band between them.

page 795 note † As the parallel, though opposite, example in spectroscopy of the importance of the intense illumination of the thing looked at, than the thing looked through, to bring about the interference of waves and inversion of the simple appearance of the light of the nearer substance;—if we hold a spirit lamp burning salt between a spectroscope, and a dull distant object giving a weak day-light spectrum, no inversion takes place; and the salt lines of the lamp appear bright, utterly hiding thereby the dark, but only grey, D lines of the clouds.

But if we hold the same spirit lamp between the same spectroscope and the Sun, the bright salt lines of the poor little lamp are inverted instantly and add black, not white, to the already very dark Solar D lines.

page 796 note * Shortly after the above was written I had the opportunity in Lisbon, with a bottle of this Judson's green to my eye, to see agaves, palm-trees, acacias, Indian corn, myrtle bushes, and oleanders, not green, but of every description of red from vermilion and cochineal, to deep Indian-red. Yet the green-sea water of the Tagus, absolutely refused to alter; though yellow-brown, unpainted oars, dipped into it, came up blood-red at every stroke !

In the Bay of Biscay, where the dull blue waves were running rather high under a cloudy sky, and brown sea-weed, like coils of hempen whale line, was floating about,—the bottle showed the waves of an ultra dark, sombre, almost threatening black-green, but the sea-weed of a magnificent coral red, almost luminous in the splendour and glory of its redness.

page 798 note * Readers will please to note the different meaning here attached to this word, as compared to that in which we have ourselves been employing it spectrally, and must employ it again.

page 799 note * I made several coloured sketches this last summer in Lisbon, first as seen direct or simply by my natural tri-chroic eye, and then as seen by looking through the dark-green di-chroic bottle: and if the trees at an old mill door, did stand up in this latter case like gigantic red corals, there were magnificent shades of dove-coloured gray behind, and pale sea-green sky above them to satisfy the most fastidious eye as to abstract theory, and splendour of effect, though not as to ordinary terrestrial experience.

page 801 note * From to according to the brightness of the sunlight at the time, is the strength of solution which has most effect in turning the greenest grass into the most brilliant scarlet. Stronger solutions as and both stop too much light, and make green grass only dully orange or buff.

page 802 note * Of 192 Tottenham Court Road, London.

page 804 note * In my own spectroscope, this little flame being 12 inches from the illuminating arrangement has its image re-formed there by a common magnifying glass of 3 inches solar focus, placed half-way between.

page 804 note † As this metal, or rather its alloy with Platinum, as Iridium-platinum, promises to fulfil a most important part in forming the “finder” and “reference-spectrum's” small mirror in front of a spectroscope's slit, as well as to serve for ruling diffraction “gratings” upon,—the following scrap of recent practical information about it, from Mr Hilger in his workshop, may perhaps usefully occupy this last corner of the page:—

“Iridium-platinum,” he writes in March 1879, “is a very fine metal, but rather very dear. You may easily calculate what a small mirror of one or more inches would cost, as the ounce of it is charged thirty-five shillings !! The specific gravity is about the same as platina. A piece I bought the other day, inch thick by 1·5 inch square, rather full in size, came to £13. I almost fainted at such an awful price: still it was ordered, and I had to pay for it. So far as polish is concerned, it does get as white and bright as a silver mirror, and never tarnishes in the least. I have already polished several small mirrors of iridium-platinum, and it answers perfectly.”