the Nicol N be turned round, these two overlaps will behave in respect of colour exactly as did the images O and E when QP was alone used. We may, in fact, form a Table thus :

Image. 00+EO

OE+EE

Colours extinguished.
B+A+B'+A-B+B'+A=A

B'+A'+B+A'=B+B'+A'=A'.

And since the tints B, B' have disappeared from each of these formulæ, it follows that the second analyzer P may be turned round in any direction without altering the tints of the overlaps in question. In like manner we may form the Table

00+EE
OE+EO

B+A+B+A'=B+A+A'=B
B'+A'+B'+A=B'+A+A'=B'.

Hence if the Nicol N be turned round, these overlaps will retain
their tints; while if the analyzer P, be turned, their tints will vary,
although always remaining complementary to one another.
There remains the other pair of overlaps, viz. :-

Each of these is deprived of the pair of complementaries A, A', B, B'; and therefore each, as it would seem, ought to appear white of low illumination, i. e. grey. This effect, however, is partially masked by the fact that the dark bands are not sharply defined like the Fraunhofer lines, but have a core of minimum or zero illumination, and are shaded off gradually on either side until at a short distance from the core the colours appear in their full intensity. Suppose, for instance, that B' and A' were bright tints, the tint resulting from their suppression would be bright; on the other hand, the complementary tints A and B would be generally dim, and the image B+A bright, and the overlap B+A+B'+A' would have as its predominating tint that of B+A; and similarly in other

cases.

There are two cases worth remarking in detail, viz., first, that in which

B=A', B'=A,

i. e. when the same tints are extinguished by the combination Q P and by Q, P. This may be verified by either using two similar quartz plates Q, Q, or by so turning the prism P, that the combination Q, P, used alone shall give the same complementary tints as QP when used alone. In this case the images have for their formulæ the following:

in other words, O O and EO will show similar tints, and EO, EE complementary. A similar result will ensue if B=A, B'=A'.

Again, even when neither of the foregoing conditions is fulfilled, we may still, owing to the breadth of the interference-bands, have such an effect produced that sensibly to the eye

and in that case

B+A=B'+A';

B'+A=B+A—A'+A

= B+A'+2A-2A',

which imply that the images O O and OE may have the same tint, but that EO and EE need not on that account be complementary. They will differ in tint in this, that EE, having lost the same tints as EO, will have lost also the tint A, and will have received besides the addition of two measures of the tint A'.

Effect of Combinations of two Colours.-A similar train of reasoning might be applied to the triple overlaps. But the main interest of these parts of the figure consists in this, that each of the triple overlaps is complementary to the fourth single image, since the recombination of all four must reproduce white light: hence the tint of each triple overlap is the same to the eye as the mixture of the two tints suppressed in the remaining image; and since by suitably turning the Nicol N or the prism P,, or both, we can give any required position to the two bands of extinction, we have the means of exhibiting to the eye the result of the mixture of the tints due to any two bands at pleasure.

Effeci of Combinations of three Colours.-A further step may be made in the combination of colours by using a third quartz, Q2, and a third double-image prism, P2, which will give rise to eight images; and if C C be the complementaries extinguished by the combination Q, P, the formula for the eight images may be thus

written :

The total number of combinations of tint given by the compart

ments of the complete figure will be :—

The most interesting features of the figure consist in this, that the subjoined pairs are complementary to one another, viz. :

And if the prisms P, P,, P, are so arranged that the separations due to them respectively are directed parallel to the sides of an equilateral triangle, the images will be disposed thus :

:

The complementary pairs can then be read off, two horizontally and two vertically, by taking alternate pairs, one in each of the two vertical, and two in the one horizontal row; and each image will then represent the mixture of the three tints suppressed in the complementary image.

Low-tint Colours. -A slight modification of the arrangement above described furnishes an illustration of the conclusions stated

by Helmholtz, viz. that the low-tint colours (couleurs dégradées), such as russet, brown, olive-green, peacock-blue, &c., are the result of relatively low illumination. He mentioned that he obtained these effects by diminishing the intensity of the light in the colours to be examined, and by, at the same time, maintaining a brilliantly illuminated patch in an adjoining part of the field of view. If therefore we use the combination N, Q, P, P, (i. e. if we remove the second quartz plate), we can, by turning the prism P round, diminish to any required extent the intensity of the light in one pair of the complementary images, and at the same time increase that in the other pair. This is equivalent to the conditions of Helmholtz's experiments; and the tints in question will be found to be produced.

"Further Experiments on the Transmission of Sound." By John Tyndall, D.C.L., LL.D., F.R.S., &c.

The author describes a number of experiments made with heterogeneous atmospheres obtained by saturating alternate layers of air with the vapours of various volatile liquids. Starting from his observation on the transmission of sound through a snow-storm on the Mer de Glace, in the winter of 1859, he shows the extraordinary power of sound to pass through the interstices of solid bodies as long as the continuity of the air is preserved. Sound, for example, penetrates through twelve layers of a silk handkerchief, while a single layer of the same handkerchief dipped into water, so as to fill the interstices, cuts off the sound.

He also describes numerous experiments with artificial fogs of a density so great that a depth of three feet sufficed to intercept the concentrated beam of the electric light; the effect of such fogs on sound was sensibly nil. Experiments were also executed on the illumination of such fogs by sudden flashes, obtained by the combustion of gunpowder or gun-cotton, or by the alternate extinction and revival of the electric and other lights. Such flashes promise to be extremely useful as fog-signals.

The author corrects the mistake of supposing that, in the experiments at the South Foreland, the lower trumpets were not compared with the higher ones. This, in fact, was the first step of the inquiry.

He also communicated an extraordinary instance of the interception of sound during one of the battles of the late American war. In these experiments the author has been ably aided by his assistant, Mr. John Cottrell. An account of the experiments will be found in a paper now printing for the Philosophical Transactions.

GEOLOGICAL SOCIETY.

[Continued from p. 77.]

February 25, 1874.-John Evans, Esq., F.R.S., President, in the Chair.

The following communication was read:

"Note on the occurrence of Sapphires and Rubies in situ with Corundum, at the Culsagee Corundum Mines, Macon Co., North Carolina." By Col. C. W. Jenks.

The mine described in this paper is in a hill situated about nine miles east of Franklin, the chief town of Macon County, rising about 400 feet above the valley. The hill is a boss of serpentine protruded through the surrounding granite. The corundum occurs in five nearly parallel veins, cropping out for about a mile along the steep side of the hill in a direction N.E. and S.W. The veins all dip to the S.E. about 45°. They are thin at the surface, but widen out as they descend, the thickness of the vein in the deepest working (75 feet) being about 10 feet. They consist of a mass of chlorite, Jefferisite, and corundum, the latter forming from one third to one half of the mass, and occurring in crystals imbedded in the other minerals. The author gives a list of several minerals which also occur in the veins, including two new silicates, which Prof. Genth has called Kerrite and Maconite. Analyses of some of these minerals and of the serpentine rock are appended to the paper. Some of the crystals of corundum weigh as much as 300 pounds. The corundum is crushed and used for grinding and polishing stones, glass, and metal; about 200 tons have been extracted from the mine. The colour of the crystals is very variable; and some of them show different colours in different parts. Many rubies and sapphires have been already procured and cut for setting.

March 11th, 1874.-John Evans, Esq., F.R.S., President,

in the Chair.

The following communications were read:-

1. "On the relationship existing between the Echinothuridæ, Wyville Thomson, and the Perischoechinidae, McCoy." Etheridge, Esq., jun., F.G.S.

By R.

In this paper the author referred in the first place to the peculiar characters of the genera Calveria and Phormosoma, Wyville Thomson, and especially to those in which they approach the Cretaceous genus Echinothuria, S. P. Woodward, and which led Prof. Wyville Thomson to include these three forms in his group Echinothuridæ. He remarked that an overlapping of the interambulacral plates, more or less like that occurring in these three genera, is met with also in Archæocidaris, McCoy, and Lepidechinus, Hall, belonging to the group of paleozoic Echini which McCoy proposed to call Perischoechinidae, and which is characterized by the presence of more than three rows of plates in the interambulacral areas. As