“The majority of shells of the lowest zone are white or transparent; if tinted rose is the hue, a very few exhibit markings of another colour. In the seventh region, white species are also very abundant, though by no means forming a proportion so great as the eighth. Brownish red, the prevalent hue of the brachiopoda, also gives a character of colour to the fauna of this zone; the crustacea found in it are red. In the sixth zone the colours become brighter, reds and yellows prevailing,—generally, however, uniformly colouring the shell. In the fifth region many species are banded or clouded with various combinations of colours, and the number of white species has greatly diminished. In the fourth, purple hues are frequent, and contrasts of colour common. In the second and third, green and blue tints are met with, sometimes very vivid; but the gayest combinations of colour are seen in the littoral zone, as well as the most brilliant whites.
“The animals of Testacea, and the Radiata of the higher zones, are much more brilliantly coloured than those of the lower, where they are usually white, whatever the hue of the shell may be. Thus the genus Trochus is an example of a group of forms mostly presenting the most brilliant hues both of shell and animal; but whilst the animals of such species as inhabit the littoral zone are gaily chequered with many vivid hues, those of the greater depth, though their shells are almost as brightly covered as the coverings of their allies nearer the surface, have their animals, for the most part, of a uniform yellow or reddish hue, or else entirely white. The chief cause of this increase of intensity of colour as we ascend is, doubtless, the increased amount of light above a certain depth.”—p. 172.
[92] Ἁμὁρφωτα. On the Epipolic Dispersion of Light, being a paper entitled, On a case of Superficial Colour presented, by a homogeneous liquid internally colourless. By Sir J. F. W. Herschel, Bart, K.H., F.R.S., &c.—An epipolized beam of light (meaning thereby a beam which has once been transmitted through a quiniferous solution, and undergone its dispersing action) is incapable of further undergoing epipolic dispersion. In proof of this the following experiment may be adduced,—
A glass jar being filled with a quiniferous solution, a piece of plate glass was immersed in it vertically, so as to be entirely covered, and to present one face directly to the incident light. In this situation, when viewed by an eye almost perpendicularly over it, so as to graze either surface very obliquely, neither the anterior nor posterior face showed the slightest trace of epipolic colour. Now, the light, at its egress from the immersed glass, entered the liquid under precisely the same circumstances as that which, when traversing the anterior surface of the glass jar, underwent epipolic dispersion on first entering the liquid. It had, therefore, lost a property which it originally possessed, and could not, therefore, be considered qualitatively the same light.—Philosophical Transactions, vol. cxxxvi.
[93] In connection with this view, the Newtonian theory should be consulted, for which see—A Letter of Mr. Isaac Newton, Professor of the Mathematicks in the University of Cambridge; containing his new Theory about Light and Colors: sent by the Author to the Publisher, from Cambridge, Feb. 6, 1671–72, in order to be communicated to the Royal Society.
[94] In that admirable work, The Physical Atlas of Dr. Berghaus, of which a very complete edition by Alexander Keith Johnstone is published in this country, the following order of the distribution of plants is given:—
| 1. The region of palms and bananas | Equatorial zone. |
| 2. Tree ferns and figs | Tropical zone. |
| 3. Myrtles and laurels | Sub-tropical zone. |
| 4. Evergreen trees | Warm temperate zone. |
| 5. European trees | Cold temperate zone. |
| 6. Pines | Sub-arctic zone. |
| 7. Rhododendrons | Arctic zone. |
| 8. Alpine plants | Polar zone. |
Consult Humboldt, Essai sur la Géographie des Plantes, Paris, 1807; De Distributione Geographicâ Plantarum, Paris, 1817. Schouw, Grundzüge der Pflanzengeographie. Also his Earth, Plants, and Man; translated by Henfrey, in Bohn’s Scientific Library. Lamouroux, Géographie Physique. The Plant, a Biography: by Schleiden; translated by Henfrey. Physical Geography: by Mrs. Somerville.
[95] Fraunhofer’s measure of illuminating power is as follows:—
| At the | 22nd degree of the red | 0·032 |
| " | 34th degree of the red | 0·094 |
| " | 22nd degree of the orange | 0·640 |
| " | 10th degree of the yellow | 1·000 |
| " | 42nd degree of the yellow | 0·480 |
| " | 2nd degree of the blue | 0·170 |
| " | 16th degree of the indigo | 0·031 |
| " | 43rd degree of the violet | 0·0056 |