There is another case of coloration which, I believe, has not yet been noticed, and which admits of a similar explanation. If a sheet of paper, with black characters, either printed or written, be moved rapidly backwards and forwards at the ordinary distance of distinct vision, the lines described by the motion will appear accompanied by very evident colours, the green and red obviously predominating. The experiment succeeds better if the lines are far apart, and perpendicular to the direction of the motion; and is still more perfect if a printed word be fixed at the extremity of a vibrating wire, (as mentioned in the description of the Kaleidophone, in the Journal of Science, N.S., No. xxiii. p. 344). This experiment indicates that there is a faint production of colours at the limits of light and darkness.

From all the known facts, it may be inferred that luminous impressions, continued on the same part of the retina, are evanescent in proportion to their feebleness; and that there are two means by which weak objects may be rendered continuously visible: 1st, by shifting their positions on the retina, and 2ndly, by causing them to act intermittently on the same points of the retina.

Though these are obvious inferences from the collected observations above stated, some of the facts separately presented might appear to admit of other explanations. Thus Majendie and Desmoulins concluded, from the circumstances noticed by them, that the sum of a certain number of impressions on different points of the retina in a given time may render a body visible, which would not be so were the interval of the impressions greater, or their number not sufficient.' This explanation would answer only for a limited number of the facts now brought together, and would exclude the experiments of Prévost, where the image is periodically presented to the same part of the retina.

There are various other optical, or rather visual phenomena which equally manifest the truth of the inferences above drawn; but as they are complicated with other circumstances foreign to the present purpose, viz., illustrating the explanation formerly given of the vascular figure of the retina, the consideration of them will be deferred to a future occasion.

C. W.

ON THE RIPPLE-MARKS AND TRACKS OF CERTAIN ANIMALS IN THE FOREST MARBLE.

By G. POULETT SCROPE, Esq., F.R.S., F.G.S., &c.

THE surface of the great elevated oolite range north of Bath is occupied throughout a very considerable area, by highly fissile limestone beds, belonging to the forest marble, and a prolongation of the Stonesfield slate, which are here likewise in general use for roofing buildings. Residing in the centre of this district, I have had frequent opportunities of observing, in a great number of neighbouring quarries, the tendency of this rock to exhibit a wavy or wrinkled surface, so completely identical in all its varieties with the rippled markings of the sea-sands left dry by the ebbing of the tide upon some of our coasts, as to leave no room for doubting that it was produced precisely in the same manner, at the period of the deposition of the beds.

This configuration, though it has not yet perhaps attracted sufficient attention, (suggesting as it does several very interesting questions, and tending to confirm many important geological views,) has been remarked by others as well as myself, and in other localities. But I have also lately discovered other appearances on the rippled surface of these beds, of a novel character, to which it may be worth while to call the attention of those geologists who have time and opportunity for the further examination of this and similar marine formations.

I have observed the ripple-mark in a vast number of quarries, scattered pretty thickly over a broad band of country, stretching along the eastern slope of the great oolite range from Bradford in Wilts, to Tetbury in Gloucestershire. I have little doubt that it will be found elsewhere along the continuation of the same beds.

It is repeated throughout a series of strata of considerable thickness; and is continuous, not only over slabs of the largest size which the quarry-men uncover at once, (I have seen one twenty-five feet long entirely covered with these wrinkles,) but apparently extends throughout a very much greater area, to be

measured, perhaps, in miles; the corresponding beds in neighbouring quarries being found to have the same configuration.

It is to be seen chiefly in the very fissile laminæ, but not unfrequently on the surface of slabs eight or ten inches in thickness. It affects indifferently those which contain a large proportion of clay, those which are highly calcareous and crystalline, and others in which sand and oolitic grains, or minute fragments of shells predominate. The only circumstance, as it appears to me, which the ripple marked beds possess in common, is their separation from the neighbouring strata, bý more or less thin seams of clay, moulded on the irregular surface below, and by which the preservation of that surface, in complete integrity, exactly as it was figured by the waves of the ocean, at an incalculable distance of time, seems to be simply and naturally accounted for.

The ripple-marks are always on the upper face of the bed; but where the seams of clay are very thin, the alternating limestone laminæ have taken the impression of the uneven surface on which they were deposited, and thus present an imperfect ripple on their lower face also. In this case, however, the undulations of the upper and lower surfaces do not correspond, but often cross and run counter to one another: occasionally, too, a double system of wrinkles may be seen on the same surface; the undulatory movements of the water, by which they were produced, having shifted their direction (perhaps through a change of wind) during the period of the deposition.

I am not acquainted with any published explanation of the cause of the rippled surface which, at low tide, may be seen extending over many square miles of sand or mud along the Devonshire, Lancashire, and many other of our flat and shallow shores. That it is disturbed and renewed again, partially or entirely, by every fresh tide, is known to all who have remarked the constant changes which it undergoes, and the obliteration of all marks made in the sand at one low tide, before the next ebb. There can be little doubt that it is produced by the oscillatory motion of the lower stratum of water in contact with the sandy or muddy bottom, as communicated to it from the

superficial waves. It is easily imitated by agitating to and fro a vessel of water, with a flat bottom, on which sand has been strewed.

To what depth superficial undulations affect water is a problem yet unsolved, though the general opinion is, that they do not ever extend beyond thirty or forty feet. The most violent movements of the surface water must be neutralized by its inertia, and their lateral extension as they are propagated downwards. But they will probably reach considerable depths before they die away entirely, and will then, I conceive, subside in precisely the sort of gentle and minute oscillations fitted to produce the wrinkles in question in mud or sand, either in the act of subsiding to the bottom, or stirred up there by the commencement or increase of the agitation.

There is an observation which, it strikes me, might perhaps be applied to determine the depth to which the superficial undulations of water are propagated; namely, by ascertaining the depth of the water at the spot where the waves, approaching a shore or shoal, begin to swell above their average height in deep water, and to take the line of direction of the shoal or the coast, instead of that which the wind impressed them with originally. Supposing the British Channel wrinkled by waves driven before a powerful westerly wind; these waves, which in mid-channel have their long axes directed due north and south, will, as they near the coast on either side, but particularly the shoaling coasts of Devon and Dorset, gradually take the line of the shore, upon which each wave breaks at length in a direction nearly exactly parallel with all its sweeps. This alteration of their original direction is, no doubt, impressed on them by their reaction from the bottom, the resistance of which retards the waves as soon as they come in contact with it, and gradually compels them to assume its sweep. The reaction of each oscillation from the bottom also causes it to rise by the rebound higher at the surface, and hence the swell of each wave as it nears the shore, and its beautiful curve and fall at last, owing to its superficial movement outstripping that of the lower part, which is retarded by the friction of the bottom.

A series of careful observations on the depth of the water at

which waves of given heights, under similar circumstances of wind and current, begin to swell and conform to the direction of the shore, would afford reasonable data for determining the depths to which the undulatory movement is propagated; and I throw out this as a hint to such persons as have the requisite opportunities for making such observations, and are interested in solving this question.

A wave is clearly a parcel of water heaped up by the concussion between any external impulse and the resistance of its own inertia. If the impulse is single, as when a stone is thrown into still water, the disturbance subsides through a succession of oscillations, like those of a pendulum, till the equilibrium is restored. The wave formed in the parcel of water first affected, as it descends, communicates the impulse laterally to the next parcel, which consequently rises, and falling again transmits the impulse to a third, and so on, until the original impulse is lost by expansion. If the impulse is continued, as by the prolonged action of wind on the surface of water, the waves maintain their full force, or rather increase gradually, so long as the fresh impulse received during each oscillation is greater than that lost by lateral or vertical dispersion. Hence, when a wind begins to act on a calm surface of water, the waves, at first small, gradually wax higher and broader, and no doubt progress downwards in the same ratio; and what sailors call a swell' gets up, after a wind has blown on the sea for a certain time. This swell continues, owing to the vast momentum acquired by the agitated waters, long after the wind which produced it has gone down or shifted, and gradually subsides as it gradually commenced. It is often vulgarly supposed that

there is a real movement of the water in the direction of the waves, and indeed the eye has some difficulty in detecting that this is not the case. On the contrary, waves caused by wind frequently move with great rapidity in the opposite direction to that in which the body of the water is carried by tides or currents.

The long axes of waves are of course transverse to the impelling force. In the annexed diagram, each of the waves alternately rises and falls.