banks and contain shells of existing species, while a more trenchant line of demarkation is nowhere seen than between these periods, as we might expect to be the case where the long interval of the Pliocenes has passed between them.

The Miocene line at Shark river is distinguished by a thin layer of calcareous earth replete with the teeth of several species of shark, of a pale ochreous color, whilst those of the Eocene are black; over the layer above mentioned is about six feet of dark Miocene marl, nearly or quite destitute of organic remains.

The upper part of the Eocene is a hard gray rock, about four feet thick, abounding in large green grains, and holding numerous fossil remains, among which Aturia is most characteristic of the geological age of the formation; below this of unknown depth is the loose green-sand with a few Eocene species of reptiles, mammals, fishes and plants.

ART. XXXV.-On certain Phenomena of Transmitted and Diffused Light; by M. CAREY LEA, Philadelphia.

white screen at

WHEN a beam of sunlight is thrown upon the distance of fifteen or twenty feet and a plate of finely ground glass is interposed in its path, the white light by passing through the colorless glass acquires a deep orange yellow coloration. A greenish or bluish tinge in the glass does not interfere with the experiment, but it is necessary that the grinding of the glass be exceedingly fine, the surface must be scarcely removed, and with the finest emery.

This experiment, which admits of some very interesting variations that will be mentioned farther on, is, I believe, new. Those upon record which approach most nearly to it, relate to certain properties of milk and of magnesia respectively, which when diffused through water produce a reddish diffuse transmitted light, and a bluish diffuse reflected light.

My own experiment just mentioned, differed from these in two points; first, that the red transmitted light was direct, and not diffuse; and secondly, that the blue diffuse light was wholly wanting. Nevertheless, a study of the phenomena led me to the conclusion that these several experiments as well as many other new ones, depended upon the same cause, and that consequently the explanation given for the few cases that have been up to this time observed and described, is insufficient.

Becquerel in speaking of the two cases just mentioned, viz: magnesia and milk, observes in his very valuable work on Light:

"La diffusion qui est tres-forte pour les rayons très-réfrangibles fait paraitre ces corps blancs et même bleuatres par reflexion, et jaunatres par transmission. En effet la lumière transmise comprend celle qui échappe à la diffusion par reflexion ainsi qu'à l'absorption; plus cette diffusion est grande, plus la partie est faible à égalité d'absorption, et vice versa; en outre la couleur des rayons diffusès est complémentaire de celle des rayons transmis."

This indeed can scarcely be called an explanation, it is rather a re-statement of the facts in a more general form and simply affirms that the red rays have a greater tendency to be transmitted and the blue to be diffused. Nor does this seem to have been deduced from any observed properties of light, but appears to have been adopted from an analogy real or supposed, with properties of heat. It will be necessary therefore to glance for a moment at these properties of heat.

In 1840, Melloni showed the necessity of admitting the existence of a diffusive power in heat, from overlooking which, a large portion of the experiments recorded up to that time, were rendered inconclusive, especially those of Leslie, some of which till then, had been looked upon as fundamental in establishing the laws of heat. Up to that time, when heat fell upon any surface, it was held to undergo specular reflection, transmission and absorption. Melloni added to these, diffusion.

To enter at large into his experiments would take up too much space here. The essential point is, that, taking rays of heat, chiefly of high refrangibility, issuing from a lamp, he sifted out from these the rays of less refrangibility by causing them to pass through a lens, which at the same time, rendered them nearly parallel. These rays were then allowed to fall upon a disc of pasteboard covered with white lead. A thermoscope placed so as not to receive rays specularly reflected, was nevertheless powerfully deflected. To ascertain whether or not this deflection was produced by radiated heat, he interposed a screen of glass between the disc and the thermoscope; scarcely any diminution of the deflection followed. Had the rays been absorbed and radiated, they would have been of low refrangibility and would have been to a large extent, stopped by the glass screen. As they were not so stopped they were evidently rays of high refrangibility issuing from the lamp. The thermoscope being carried around the disc at the same distance and angle indicated an unvarying deviation. This showed that the effect observed could not arise from specular reflection.

It therefore followed that the deviation of the thermoscope was due to a function of heat different from any previously recognized, distinguished from specular reflection by the fact

* Becquerel, La Lumiere, ii, 10.

that it exerted itself in all directions equally while specular reflection took place in the plane of incidence only; distinguished from radiation by the fact that it is not like the latter, preceded by absorption.

Different substances showed great differences in the energy of their diffusion. Smoke black diffused all rays equally, but very little. Some diffused largely, and all rays equally; this was the case with the metals. Others again diffused rays of high refrangibility much more than those of lower refrangibility.

In seeking analogies between heat and light, Melloni compared this new function of heat with that species of irregular and diffused reflection in light which conveys to our senses the conception of color. Smoke black he observed was truly black as respects heat, as well as light, diffusing in both cases very little, but rays of all refrangibilities equally. The metals, in their relations to heat, corresponded with bodies white as respects light, as they diffused freely, and all rays equally. Those bodies which exercised an election to the rays, absorbing more of some, and diffusing more of others, he compared to bodies which are colored as respects light. This heat color he termed thermochrosis.

This analogy however, though striking, is far from perfect. For those bodies which exhibited an election, always absorbed the less and diffused the more, refrangible rays of heat. If the phenomena of light had been strictly analogous, we should on the one hand have had only black, white, and blue bodies. Or, on the other hand there should be bodies found absorbing the more, and diffusing the less refrangible rays of heat; these would have been analogous to bodies red, yellow and green, as respects light.

This defect in the analogy is not alluded to by Melloni. And of late years this function of heat has been, as appears by the quotation already made, compared to one of light distinct from the analogy of color imagined by Melloni, viz: to the property alleged to exist in milk and magnesia, of being red by transmitted and blue by diffused rays.

It will be observed how extremely unsatisfactory is this whole explanation. The appearances alleged to exist in milk and magnesia are said to arise from the tendency of blue rays to diffusion and red rays to transmission, without any reason being assigned for this tendency. Its existence is assumed to be proved by Melloni's experiments on heat. On examining Melloni's statements we find that he says nothing about the greater tendency of the less refrangible rays of heat to be transmitted, but only to be absorbed. Again, he considered the phenomena which he described to correspond with a set of light-phenomena

entirely different. And farther, he attempted no explanation of the elective tendencies of these bodies as respects heat, but simply recorded the fact.

In the present paper I purpose to examine more particularly than has been heretofore done, the phenomena above spoken of, and to show that they have a very wide extension, and present very varied effects. That the appearances heretofore observed and many analogous new ones which I shall here describe, depend altogether upon Interference, and under that view are susceptible of a satisfactory explanation.

These phenomena present themselves in three different aspects, which may be classified as follows:

CASE FIRST.-A strong beam of yellow, red, or reddish yellow direct light is produced without the complementary blue being visible.

CASE SECOND.-The yellow or red direct beam is visible and simultaneously the blue, the latter diffused.

CASE THIRD.-Reddish and bluish light, both diffused, are simultaneously visible.

The simple existence of this third class disproves the explanation usually received and quoted above, for that explanation affirms that the red light tends to transmission, and the blue to diffusion. I shall endeavor presently to show that in one of the very instances usually quoted, the red and blue light undergo diffusion equally.

Case first.-Production of a yellow or red beam of direct light, in the absence of blue.

Let us take an ordinary plane silvered mirror about six inches by eight, and, allowing a large beam of sunshine to pass into the darkened room, let the beam fall upon the mirror lying horizontally or nearly so, upon a table, so as to be reflected upon the ceiling or a white wall, fifteen or twenty feet distant.

Let us now interpose in the path of the ray after its reflection from the mirror a plate of very finely ground glass. The ray which now passes through the colorless glass is no longer white, but of a deep yellow color. It is necessary in order that this experiment should fully succeed, that the grinding should be extremely fine. Ordinary ground glass will not answer at all.*

As it is extremely difficult to procure glass ground finely enough, it is fortunate that the same result can be produced in many different ways. One of the best, is the imprisonment of

The glass must be merely "greyed," not ground, and with the finest emery.

gelatinous alumina in a film of collodion. This is accomplished as follows:

A salt of aluminum soluble in alcohol is selected, the bromid is that which I have used. A few drops of strong alcoholic solution of the aluminum compound is added to ordinary plain collodion (made by dissolving a drachm of photographic pyroxylin in six ounces of ether and as much alcohol) and to this a very few drops of strong liquid ammonia are added, and, after shaking, a little ether. After standing a day, this mixture can be extended over glass, and after it is set, the bromid of ammonium is to be washed out, and the film dried.

Such a plate placed in the path of the ray colors it bright orange yellow. If it be laid upon the mirror so that the ray passes through it twice, viz: before and after reflection, the beam will be colored orange red.

Thin strata of bromid of silver upon glass produce a similar effect.

My own views as to the nature of this coloration which will be presently explained, led me to the conclusion that a precisely similar result would be produced by any substance of a sufficiently fine state of division, irrespective of its color. Experiment confirmed this expectation and led to a remarkably striking experiment. Colorless white light was dyed deep orange red by passing through a bright blue film.

To make this experiment in a rigorous manner, both cobalt and chrome blue were rejected in consequence of their tendency under certain circumstances to transmit red rays, by reason of their two maxima of absorption. Copper was selected which is wholly free from this objection, and films of hydrated oxyd of copper were obtained by preparing a saturated alcoholic solution of chlorid of copper, and by treating it precisely as explained in the case of alumina.

Reflected sunlight transmitted through these plates was colored red. I scarcely know any experiment in optics that strike an intelligent observer with more surprise when first seen, than this. The film when observed by diffuse light is blue. White objects viewed through it are colored strongly blue, but rays of sunlight transmitted through it, become salmon-red.

Case second.-Yellow or red direct rays are produced, and blue diffuse light is seen simultaneously.

Finely divided sulphur in the amorphous condition exhibits this effect very beautifully. To obtain it, two or three drops of sulphuric acid are added to fifteen or twenty ounces of water, and then two or three drops of sulphydrate of ammonium. In five or ten minutes the sulphur separates but remains sus