in binocular vision, are believed to be in part new, and are in part modified repetitions of experiments already described by Profs. Wheatstone and Dove. They offer what seems to be decisive proof that such a successive combination of pictures point by point, however it may enter in many cases into the complex process of vision, cannot be regarded as an essential condition to the singleness and perspectiveness of the binocular perception.

1. Let a brilliant line, held in a perspective position at a convenient distance midway between the eyes, be regarded intently for a few seconds so as to produce a lasting impression on the retinæ. On turning the eyes towards a blank wall or screen the subjective impression will be seen projected against it and having the same perspective altitude as the original line. If then one eye be closed the line will appear to subside into the surface of the screen taking an inclined position corresponding to the optical projection of the original line as seen by the unclosed eye and therefore corresponding to the image formed in that eye. By opening and closing the eyes alternately, and finally directing both to the screen we are able to see the two oblique lines corresponding to these projections and their binocular resultant corresponding to the original object.

2. Let two slightly inclined luminous lines formed by narrow slits in a strip of black card-board be combined into a perspective resultant either with or without a stereoscope. Looking at this for a few seconds so as to induce the reverse ocular spectrum, and then directing the eyes towards the opposite wall of the apartment, a single spectrum will be observed having the attitude and relief of the original binocular resultant,

As a strong illumination of the lines is necessary to bring out the most striking effect, the card-board should be held between the eyes and some brilliantly white surface, as the globe of a solar lamp or a strongly illuminated cloud, care being taken to prevent the entrance of extraneous light.

3. Using the same arrangement, let the luminous lines be regarded in succession each by the corresponding eye, the other eye being shaded so that no direct binocular combination can be formed. On looking toward the wall it will be seen that the two subjective images unite to form a single spectral line having the same relief as if the lines had been directly combined with or without the stereoscope.

While the perspective image continues distinctly visible, let either eye be closed the other being still directed towards the wall. The image will instantly lose its relief and take its position on the plane of the wall as an inclined line corresponding to the subjective image in the eye which has remained open. When the subjective impressions have been sufficiently strong, it is easy to alternate these effects by projecting first the picture

proper to the right eye and then that of the left on the plane of the wall, with their respective contrary inclinations. On then looking with both eyes we see the resultant image instantly start forth in its perspective altitude.

It is hardly necessary to say that in order to obtain these effects satisfactorily even with lines very strongly illuminated, the observer should have some practice in experiments on subjective vision. In these circumstances, however, I have found the results to be perfectly certain and uniform.

The conditions of the experiments are obviously such as to exclude all opportunity of a shifting of the image on the retina. Such a shifting however is essential to that successive combination of pairs of points in the two images which on the theory of Brewster is required for the production of the single perspective resultant. Hence according to this theory the resultant spectrum in these experiments, instead of being a single line in a perspective position, ought to present the form of two lines inclined or crossing, situated in the plane of the wall without projection or relief.

In reference to the first two experiments it might perhaps be maintained that as the perspectiveness of the original line or resultant on which the eyes were converged formed part of the direct perception in the first stage of the experiment, it would be likely through association to be included also in the spectral or subjective perception. But this consideration, which at best does not impress me as of much weight, is entirely inapplicable to the conditions of the last experiment, where the eyes are in the first place impressed in succession with their respective images, and where yet when they are together directed to the wall, the perspective single resultant at once springs into view.

4. Without resorting to these troublesome efforts of subjective vision the following is a simple proof that pictures successively impressed on the respective eyes are sufficient for the stereoscopic effect. Let a screen be made to vibrate or revolve somewhat rapidly between the eyes and the twin pictures of a stereoscope, so as alternately to expose and cover each, completely excluding the simultaneous vision of the two. The stereoscopic relief will be as apparent in these conditions as when the moving screen is withdrawn.

Here there is no opportunity for the combination of pairs of corresponding points in the two diagrams by the simultaneous convergence of the optic axes through them, but at each moment the actual picture in the one eye, and the retained impression in the other, form the elements of the perspective resultant which we perceive.

5. The ingenious experiments described by Prof. Dove many years ago in which the stereoscopic effect was obtained by the

momentary illumination of the eléctric flash, furnish a further and most powerful argument against the theory of successive binocular combination here referred to.

In repeating these I have found great advantage in using one of Ritchie's improved Ruhmkorff coils having a coated jar included in the outer circuit, the intensely brilliant spark of which can be made to throw its light upon the object viewed in any direction or at any interval that may be desired.

When a twin-diagram of any simple geometrical solid was placed in the stereoscope and viewed by this momentary light it was found to exhibit the perspective resultant in most cases with a single spark, and it never failed to present it in perfection with a succession of sparks even when they followed each other slowly.

A large circular disc of brass, marked with the usual concentric striae, was placed in a position to catch the illumination and produce the peculiar intersecting lines of reflected light. At each spark the bright resultant line due to the binocular union of these intersecting lines was seen penetrating the disc and extending in a steep angle beyond and in front of it.

As, according to Wheatstone, the duration of an electric spark is less than one-millionth of a second, it can hardly be supposed that in either of these experiments the eyes have time to make the successive changes of direction required, by the theory, for the singleness and relief of the observed resultant. Not less at variance with this theory is the familiar fact that the illumination of a single flash of lightning is sufficient to give us a clear perception of the forms and positions of objects to which the eyes are for the moment directed. So the long straight spark of one of Geissler's narrow vacuum tubes, glowing for an instant in a dark room, impresses a precise perception of the altitude and place of the tube and its included line of light, and even their regular path of the long spark through the air produces a distinct perception single and faithful to its devious directions.

We may therefore conclude-first, that the perception of an object in its proper relief, or that of the perspective resultant through binocular combination in a stereoscope, or otherwise, may and most usually does arise, by direct suggestion from the two pictures impressed, without requiring the successive combination of corresponding points;—and second, that for the singleness of the resultant perception, it is not necessary that the images should fall on what are called corresponding parts of the two retinas.

The condition of single vision in such cases seems to be simply this, that the pictures in the two eyes shall be such and so placed as to be identical with the pictures which the real object would make at a given distance and in a given altitude before the eyes.

XXXVI.-Geographical Notices. No. XIV.

PROF. GUYOT'S MEASUREMENTS OF THE ALLEGHANY SYSTEM.—It is well known to the scientific men of this country that Professor Arnold Guyot of Princeton, New Jersey, has devoted a portion of his summer vacations for ten years past to the study of the different portions of the great Alleghany system which faces the Atlantic coast from Canada to Georgia. Several years ago he measured the highest peaks of the Adirondack, Green and White mountains, in the northern part of the chain, and more recently he has been at work on the southern portion of the system which is found to possess the most elevated peaks of the whole Appalachian chain. His determination of some of the highest peaks of the Black mountains of North Carolina was published in this Journal, for September, 1857.

By a private letter from Professor Guyot we learn that during last summer (1860) he has devoted two full months to further measurements in the south, in company with Messrs. Sandoz and Grand Pierre. The weather has been propitious and he has accomplished much work, having measured between one hundred and fifty and two hundred points in addition to those which were previously determined. He has extended his investigations as far as Georgia, and has seen the extremity of the Blue Ridge and the Unaka. It may now be affirmed with safety that the southern portion of the Alleghanies is better known so far as pertains to its hypsometry, than any other portion of the system. There is reason to hope that at an early day Professor Guyot will lay before the readers of this Journal, in detail, the results of his important and prolonged investigations; meanwhile the reader will be interested in the following partial summary of his observations in North Carolina.

These measurements sufficiently indicate the grand traits of structure of that loftiest portion of the Appalachian system. It may be seen that the Roan and Grand Father mountains are the two great pillars on both sides of the Northgate to the high mountain region of North Carolina, which extend between the two chains of the Blue Ridge on the east and the Iron and Smoky and Unaka mountains on the west. That gate is almost closed by the Big Yellow mountain. The group of the Black Mountain rises nearly isolated on one side in the interval between the two chains touching by a corner the high Pinnacle of the Blue Ridge, and overtowering all the neighboring chains by a thousand feet. In the large and comparatively deep basin of the French Broad Valley, the Blue Ridge is considerably depressed, while the western chain preserves its increasing height. Beyond the French Broad rises the most massive cluster of highlands, and of moun

tain chains. Here the chain of the Great Smoky mountains which extends from the deep cut of the French Broad at Paint Rock, to that, not less remarkable, of the Little Tennessee, is the master chain of that region and of the whole Alleghany system. Though its highest summits are a few feet below the highest peaks of the Black Mountain, it presents on that extent of 65 miles a continuous series of high peaks, and an average elevation not to be found in any other district, and which give to it a greater importance in the geographical structure of that vast system of mountains. The gaps or depressions never fall below 5000 feet except towards the southwest and beyond Forney Ridge, and the number of peaks, the altitude of which exceeds 6000 feet, is indeed very large. On the opposite side, to the southeast, the Blue Ridge also rises again to a considerable height, in the stately mountains of the Great Hogback and Whiteside, which nearly reach 5000 feet, and keeps on in a series of peaks scarcely less elevated far beyond the boundary of Georgia.

Moreover the interior, between the Smoky mountains and the Blue Ridge, is filled with chains which offer peaks higher still than the latter. The compact and intricated cluster of high mountains, which form the almost unknown wilderness covering the southern portion of Haywood and Jackson counties, is remarkable by its massiveness and the number of lofty peaks which are crowded within a comparatively narrow space. The Cold mountain chain, which constitutes one of its main axes, shows a long series of broad tops, nearly all of which exceed 6000 feet. Near the south end, but west of it, not far from the head waters of the French Broad, the Pigeon and the Tuckaseegee waters, Mount Hardy raises its dark and broad head to the height of 6133. Still further, to the northwest, the group culminates in the Richland Balsam, 6425 feet, which divides the waters of the two main branches of Pigeon river and of the Caney fork of the Tuckaseegee. Amos Plott's Balsam, in the midst of the great Balsam chain, which runs in a parallel direction between the two main chains, measures 6278 feet. Considering therefore these great features of physical structure and the considerable elevation of the valleys which form the base of these high chains, we may say that this vast cluster of highlands between the French Broad and the Tuckaseegee rivers, is the culminating region of the great Appalachian system.

NEW MAP OF THE ALLEGHANY SYSTEM, BY MR. E. SANDOZ. The measurements of Professor Guyot, just referred to, furnish important data for the correction as well as the completion of all existing maps of the regions which he has examined. These data, with the exception of those collected in the past summer, have been employed by Mr. Ernest Sandoz, a nephew of Prof. Guyot, and an accomplished draftsman, in the construc