flora of a different character was covering the dry land, if there was any dry land, at the carboniferous epoch?

From the thickness of some beds of coal, representing a mass of combustible matter as great at least as that which is contained in our oldest and deepest peat bogs; from the thickness and various composition of the strata which separate the beds of coal, and from the successive changes in the vegetation of the coal, it appears that the last alternative is admissible. Different hypotheses have been put forward to explain the so-called huge or gigantic vegetation of the coal-formations. But there is nothing in the carboniferous epoch authorizing the supposition that the power of vegetable life was greater than it is at our time. The combustible matter heaped in some of our peat bogs is sometimes sufficiently thick to be equivalent to the coal of a bed of four to five feet. The trees growing in our marshes or on the peat bogs are generally larger than those which have been preserved in the strata of the carboniferous measures. The Dismal Swamp is impenetrable on account of the great compactness of its vegetation. It is not an easy matter either, to get across the heaped, half prostrated or erect and closely pressed trees of our cedar-swamps of the North. If such marshes were extended over the greatest part of the United States, they would present a fair representation of those of the carboniferous period.

The occasional appearance of petrified trees, standing imbedded in sandstone, does not give evidence of a rapid formation either of the coal or of the other strata. Local disturbances may throw a few feet of sand upon a marsh, covered with active vegetation, and thus preserve the stumps from decomposition and by-andby these may be converted to stone. The bald cypress and other species of trees grow sometimes in the marshes near the sea shore under ten feet of water. Whole forests of those trees have been imbedded in a standing position in the marshes around New Orleans. Thus I do not find in the geological records of the carboniferous period any indication of a rapid process of formation, either cataclysmic or abnormal, and I readily admit that each bed of coal with its accompanying strata of fire-clay and shales has required for its formation a period of time as long as any of our recent geological divisions.*

The question concerning the existence or non-existence of dry land covered with a peculiar vegetation at the epoch of the coal formation, cannot be answered positively or negatively by suffi cient evidence. The only fact that would indicate that the

Thus, if a peculiar nomenclature for a classification of the different strata of the old red sandstone, of the subconglomerate coal, and of the Millstoue-grit is admissible, the process of division should be extended to each bed of the coal

measures.

marshes of the carboniferous epoch were surrounded by landbearing plants of different kind than those living on the bogs is the presence in coal No. 1B and in the sandstone underlying it of a great number of fruits of different species which by their nature have no relation to any of the other remains preserved in the coal. They have been generally referred to species of Cordaites. But the two only species of our coal measures are found in abundance at geological horizons where the fruits are entirely absent. And even at coal No. 1B shales appearing entirely composed of heaped remains of leaves of Cordaites borassifolia do not contain any fruit. The species of fruit, Carpolithes Cordai Gein., referred by M. Geinitz to Cordaites borassifolia, our most common and omnipresent species, has not been found in the coal measures of America. Therefore, either the fruits of unknown relation, Carpolithes, Trigonocarpa and Rhabdocarpos* belong to vegetable species which have grown on the marshes, and of which the remains, leaves and stems, have been entirely obliterated or those fruits belong to species growing out of the marshes, around them, and have been floated and thus disseminated in the shales and in the sandstones. This last opinion appears at first confirmed by a similar process of distribution of species in our deep swamps. I have already mentioned elsewheret how the hollow trunks of the bald cypress which grows in Drummond lake (Dismal Swamp of Virginia) are filled by fruits, acorns, nuts, &c., of trees which grow on the dry land near its borders. But, it is not presumable that species of fruits only could have been floated and disseminated by the agency of water without any of the branches and of the leaves of the plants to which they belong. And nowhere have the shales, covering what is called the tail of a coal bank, viz: the part abutting against a hill of sand or losing itself in sandstone, exposed any remains of plants of another type than those belonging to the true coal formation. Even where the shales of the coal are covered with remains of shells and of fishes, and consequently formed when the marshes were immersed, all the floated remains of plants which are found with those of animals belong to the common species of the coal. I believe then that the plants preserved in the shales of the coal give us a fair representation of the general flora of the carboniferous epoch, as true and as general at least as the fossil plants of the miocene represent the general flora of the tertiary period. And I suppose that if there was any dry land around the marshes, the vegetation contained only a few species different from those living on the marshes. But this last opinion is merely hypothetical.

[To be continued.]

* I consider the Cardiocarpa as the fruits of Asterophyllites and probably of some species of Calamites.

Introduction to the fossil flora of Pennsylvania, Geol. Rep. of Penn., p. 847.

ART. XXXIV.—Additional observations on the Circulation in the Eye; by OGDEN N. ROOD, of Troy University, N. Y.

In a notice which appeared in the September number of this Journal, I described a subjective phenomenon, which is seen when a bright sky is viewed through two or three plates of cobalt glass. Most persons who are in the habit of directing their attention to this class of phenomena, have even perceived faint indications of the circulatory movement with the naked eye. But in the experiment with the cobalt glass, where this motion is seen with distinctness, as well as in the faint indications of it obtained with the unassisted eye, the moving bodies are not observed to be in compact masses, but as it were, sprinkled over the field of view.

If however the light of a spirit-lamp with a salted wick, be concentrated on the eye by means of a convex lens, 3 inches in diameter, having a focal length of about 3 inches, the bright field soon resolves itself into a mass of small, round, densely packed moving bodies, which appear light on a dark ground.

This is seen with varying degrees of readiness by different persons: some perceiving it in a few seconds, others requiring a protracted gaze of several minutes.

The moving bodies at first appear very closely packed together like fine mosaic-work, but as the view grows more distinct, their path can be traced, and the conviction is forced on the mind of the observer, that they are moving at slow uniform rates, through narrow channels; the whole reminding one strongly of the circulation seen in the web of a frog's foot, by a microscope slightly out of focus.

The interposition of plates of yellow glass rather adds to the distinctness of this appearance.

The indistinct vision attendant on faintness or on mere eyeweariness, is, I suspect, in many cases, mediately caused by the appearance of this circulation: for example, some weeks ago, I tried the experiment of binding up the right eye, and using only the left, for two days. After reading with left eye for half an hour, the page grew indistinct, the letters apparently dissolved and this dense circulation, which I had previously observed, set in, and continued to be visible for some minutes. It reappeared a number of times during that, and the following day.

Subjoined is an extract of a letter from Prof. Wm. B. Rogers. "Your experiment with the blue glass appears to render much more distinctly visible a phenomenon which I have often observed and which has been noticed by others. After a continued effort of vision in experiments on binocular combination or in the use

AM. JOUR. SCI-SECOND SERIES, VOL. XXX, No. 90.-NOV., 1860.

of the microscope as well as after active exercise, or any other stimulus to the circulation, I am able to reproduce it easily and with great clearness.

"When in these conditions I look intently with the naked eye upon a bright surface as that of a white cloud or a sheet of letter paper in the sunlight, the first appearance presented is that of numerous bright points darting around in various broken curves, coming in view and disappearing fitfully, but in such positions as to indicate the recurrence of the same motions, or the passage of successive particles in certain prescribed and permanent channels.

"As the eyes continue to be fixed on the surface, a shade comes over it, and on the dark ground I see innumerable streams of particles, moving in infinitely various loops and other curves, which, by a little attention, are observed to maintain a constant pattern, strikingly analogous to the capillary circulation as seen in the microscope. These streams of particles are of a tawny yellow tint well contrasted with the dark brownish surface in which they appear.

"Under the most favorable conditions of the organ I can usually obtain the effect with both eyes open, but it is more certain and far more distinct with a single eye, the other being closed. I should add that the appearance rarely lasts more than one or two seconds at a time, being obliterated by the recurrence of the white illumination and after a short interval returning, though usually with less distinctness than when first seen.

"In looking through a tube of black paper at a white surface until the eye has become fatigued, and we should perhaps add greatly excited, I have often noticed the same phenomenon and I believe I could at any time bring into view the yellowish mass of the circulation by continuing the experiment for a minute or two. With the lens of my pocket microscope held at about the focal distance I scarcely ever fail to obtain this effect in a few seconds.

"Your experiment with the blue glass is very interesting and succeeds with me perfectly. It will be a great gain if with comparatively little discomfort and without risk we shall be able to scrutinize steadily this intraocular phenomenon.'

ART. XXXV.-Some experiments and inferences in regard to Binocular Vision; by Prof. WILIAM B. ROGERS.

IN the theory of binocular vision which has been so ably expounded by Sir David Brewster, Brücke, and others, it is contended that no part of an object is seen single and distinctly, but that to which the optic axes are for the moment directed, and that "the unity of the perception is obtained by the rapid survey which the eyes take of every part of the object." According to this view our perception of an object in its solidity and relief, instead of being the simple and direct result of the pictures formed at any one moment in the two eyes, is acquired by a cumulative process, in which the optic axes are successively converged upon every point of the object within view.

*

In an experimental discussion of some points in binocular vision published in the Am. Journ. of Science several years ago, I was led to conclude that the phenomena of the stereoscopic resultant do not necessarily or even usually conform to these conditions, and that "the perception of a perspective resultant line or of a physical line in the same attitude does not require the successive convergence of the axes to every point." The grounds of this conclusion were,-first that the perspectiveness of the resultant although not perceived when the axes are steadily maintained at any one convergence, appears as soon as they are allowed sufficient freedom of motion to unite a few contiguous points of the component lines, and that it then effects the whole extent of the resultant; and-second that the resultant presents a perspective attitude even when the component lines, instead of being united into one, are brought together so as to intersect at a small angle, each of the intersecting lines in this case appearing in relief.

Satisfied from these considerations that the perceived singleness and relief of the resultant are to be ascribed rather to a process of suggestion then to the exhaustive binocular survey which has been supposed necessary, I was led to the opinion that, while some change of direction and convergence of the axes is needed for the effect, the successive view of a few contiguous points is all that is required to bring the suggestive action into play and to give rise to the full perception of the position and relief of the resultant.

In this view while rejecting the theory of successive vision in the form in which it has been propounded, I still considered some degree of motion of the axes as one of the steps by which we obtain the perception of the binocular resultant.

The following experiments, intended still further to test the theory of the successive combination of corresponding points

*This Journal, [2], xx, 86, 204, 318, and xxi, 80, 173, 439.