man and Bannister, relate more especially to the geology of various counties of the State, and contain much interesting local and general information.

Part II, (pp. 291-574,) is a valuable contribution to American Paleontology, and reflects much credit on its authors, Messrs F. B. Meek, and the Director of the survey. Many new species of paleozoic fossils are here described, and among them, the interesting Articulates from the Coal-measures will especially attract the attention of paleontologists. An important Supplement, by Mr. S. H. Scudder, containing descriptions of fossil insects from the Coal measures of the State, and another by Dr. J. V. Z. Blaney, giving the results of chemical analyses of various limestones, complete the work, which forms a worthy companion volume to those already issued.

PROCEEDINGS ACAD. NAT. SCI., PHILADELPHIA, No. 2, March, April, May, 1868. p. 92, Remarks on fragments of a large Enaliosaurian; E. D. Cope.—p. 93, Remarks on the New Species of Osmerus; T. Norris.-p. 94, Unionidæ from Lake Nicaragua; I. Lea.--p. 96, Reptilia, and Batrachia obtained by the Orton Expedition to Equador and the upper Amazon, with notes on other species, and a Second Supplement on some New Raniformia of the Old World; E. D. Cope.p. 140, Sexual Law in Acerdesycarpum; T. Meehan.--p. 142, On a New Mineral in Cryolite; T. D. Rand.--p. 143, Description of Sixteen New Species of Unio of the United States; 1. Lea.-p. 145, Notes on some Singular Forms of Chinese species of Unio; 1. Lea.--p. 147, Remains of a New Ophidian from the Green Sand of New Jersey; E. D. Cope.-p. 148, List of Birds collected at Laredo, Texas; H. B. Butcher.-p. 150, Descriptions of four New Species of Exotic Unionidiæ; I. Lea.-p. 151, Description of Twenty-six New Species of Melanidæ of the United States; I. Lea.

PROCEEDINGS BOSTON Soc. NAT. HIST. Vol. XII.--p. 81, Description and History of a New Species of Erirhinus; F. G. Sanborn.-p. 82, Eruption of Mauna Loa; W. T. Brigham.-p. 83, Projection of After-pictures; B. J. Jeffries.p. 84, Analysis of Petrosilex; C. T. Jackson -p 85, Birds of Iowa and Illinois; J. Allen.--p. 86, The Hairy Men of Yesso; W. P. Blake-p. 87, Migratory Grasshoppers of the United States; S. H. Scudder.--p. 88, Apatite at Perth, Canada West; C. T. Jackson.-p. 90, Larva and Pupa-case of Microdon globosus; F. G. Sanborn.-p. 91, Comparison of Young Larvæ of Insects; L. Trouvelot.-p. 92, Analogy between Lima-codes and some Hymenoptera; L Trouvelot-p. 94, The Thaumatrope; B. J. Jeffries.-p. 96, New Mineral Locality, Auburn, Me.; L. Hills. p. 97, Metamorphosis of Siredon into Amblystoma; O. C. Marsh-p. 99, Reproduction of lost limbs in the Walking-stick; S. H. Scudder-p. 100, On a Thread Worm infesting the Brain of the Snake-bird; J. Wyman.-p. 104, Synopsis of the Birds of South Carolina; E. Coues.-p. 128, On the Nature of the Movements involved in the Changes of Level of Shore Lines; N. S. Shaler.—p. 136, Deep Dredging between Cuba and Florida; Pourtales.-Disappearance of the Cane from the central part of the Ohio valley; N. S. Shaler.-p. 138, Nephrite from Turkistan, R. von Schlagintweit.—p. 139, Hodotermes Japonicus; H. Hagen-A Century of Orthoptera, Decade I, Gryllides; S. H. Scudder.-p. 143, Natural History of Alaska; W. H. Dall.-p. 145, Absence of Glacial Action in the valley of Youkon River; N. S. Shaler.-p. 152, Obituary of Horace Mann; W. T. Brigham. p. 157, Nautilus pompilius in Alcohol, from the Moluccas; A. S. Bickmore. p. 158, Results of Mann's study of the Hawaiian Flora; W. T. Brigham.

THE

AMERICAN

JOURNAL OF SCIENCE AND ARTS.

[SECOND SERIES.]

ART. XI.—On some phenomena of Binocular Vision; by JOSEPH LECONTE, Prof. Chem. and Geol. in University of South Carolina.

[Continued from page 77.]

II. Rotation of the eye on the optic axis.

NEARLY all the experiments described in this paper had already been made and the results obtained, when my attention was called to Helmholtz's Croonian lecture "On the normal motions of the eye in relation to binocular vision."* From this lecture I received some useful hints, as to the best method of experimenting on this subject, which have been of great service to me, and have made my results much more satisfactory, without, however, materially modifying them. As these results differ very greatly and fundamentally from those of Helmholtz, I repeated the experiments daily for many weeks, modifying them in every conceivable way, to avoid the possibility of error. I am perfectly sure, therefore, that the results are true for my own eyes, and as far as I have been able to have them verified, they are true also for most other normal eyes. Unfortunately, however, the difficulty of verification for other eyes is very great. Many of these experiments which I find perfectly easy are almost impossible for most persons.

Helmholtz's lecture, I suppose, is the most authoritative statement which we have of the present condition of science on * Proc. Roy. Soc., April, 1864, vol. xiii, p. 186.

AM. JOUR. SCI.-SECOND SERIES, VOL. XLVII, No. 140.—MARCH, 1869.

the subjects of the motions of the eye and of the Horopter. It seems to be an abstract of more extended researches which I have not seen. On this account it is obscure in some parts; yet I think I cannot be mistaken in his general results. In order to make myself clear, whether in discussing Helmholtz's results or in describing my own experiments, I find it necessary to define the terms I shall most frequently use. The position of the eye when the optic axes are parallel and at right angles to the vertical line of the face, as when with head erect we look at a point on a distant horizon, is called by Helmholtz the primary direction of the eye, and the visual line in this case is the primary direction of the visual line. All other directions are called secondary directions. A plane which passes through the visual line is called a meridian plane of the eye, and the intersection of such a plane with the retina we will call a meridian of the eye. The vertical line of demarkation is that meridian of the eye upon which the image of an apparently vertical line falls when we look directly at the line, and which therefore divides the retina into two equal halves containing corresponding points in the two eyes. The horizontal line of demarkation is that meridian of the eye upon which under similar circumstances the image of an apparently horizontal line falls. The plane which passes through the two visual lines we will call the visual plane, and that visual plane which is at right angles to the line of the face the primary visual plane. The line joining the root of the nose and the point of sight, and which therefore bisects the angle of optic convergence, we will call the median line of sight.

Now Helmholtz gives as the law controlling all the movements of the eye the following, viz: that when the eye turns from its primary to any secondary position, it turns "on a fixed axis which is normal both to the primary and to the secondary visual line." In other words, the eye may turn on any axis at right angles to the optic axis, but does not rotate about the optic axis. Again he states that "vertical and horizontal lines keep their vertical or horizontal position in the field of vision when the eye is moved from its primary direction vertically or horizontally." This law had been previously stated by Listing but without proof; Helmholtz claims to have established it by experiment. His method is very ingenious. It is well known that if we look for some time at a bright object, and then turn the eye upon a comparatively obscure field, a spectrum having the form of the object will be seen. As such spectra are the result of a temporary modification of the retina itself, they must follow the motions of the eye with the greatest exactness. If therefore the bright object be a line, then if

there be any rotation of the eye on the optic axis, in turning the eye in various directions the linear spectrum ought to incline to one side or the other. Suppose, then, the object be a bright red vertical line on a gray wall at the exact height of the eye: Helmholtz finds that on gazing at the bright line with one eye, taking care that the eye shall have its primary direction, and then turning the eye in a horizontal plane to the right or left, the spectrum retains perfectly its verticality. "I found," he says, "the results of these experiments in complete agreement with the law of Listing." For the ingenious device of Helmholtz for getting the primary position of the eye we must refer the reader to his lecture. I have tried Helmholtz's experiments with similar results. Nevertheless, I believe it may be demonstrated that though rotation of the eye does not take place under the circumstances of these experiments, yet it does so under other circumstances not touched by them; and that in a manner which deeply affects the question of the Horopter. The law of Listing is doubtless true or nearly true when the eyes move together parallel to each other, but is far from being true in strong convergence. The experiments which follow prove beyond a doubt that in my own case and in most other cases tried, the eyes in convergence rotate on the optic axes outward, and that the amount of rotation increases with the degree of convergence. Meissner has attempted to determine experimentally the position of the Horopter, and from the position thus determined he infers the rotation of the eyes; my experiments prove directly the rotation of the eyes, and from this as well as from direct experiment I hope to establish the position of the Horopter.

Helmholtz, it is true, admits some degree of rotation of the eye on the optic axis, particularly when the eye makes wide excursions in the field of view; but that he does not regard this as sufficient to interfere seriously with the law of Listing is evident from the form of the Horopter which he deduces. Moreover, according to Helmholtz, these slight rotations are controlled by the law of Donders, viz: "that the eye returns always into the same position when the visual line is brought into the same direction." He regards this law as rigorously exact. "Every position of the visual line," he says, "is connected with a determined and constant degree of rotation." But the experiments about to be described prove that under certain circumstances the law of Donders, too, is far from being true.

We have already stated (p. 73) that when the squares of the ruled diagram (fig. 5) are combined by converging the * Bib. Un. Archiv. des Scien., II, vol. iii, p. 160.

optic axes, if the amount of convergence be great, the horizontal lines of the two images are distinctly observed to cross each

other at small angle. After my attention was once directed to this fact, I could see slight crossing of the horizontals for every degree of convergence, but the verticals seemed to coalesce perfectly. By placing, however, both the diagram and the head perfectly perpendicular, looking straight forward at a point exactly at the same height as the eyes, the visual plane therefore in the primary position, and then slowly increasing or decreasing the convergence of the optic axes, so that the vertical lines of the two images passed slowly over one another, it was plainly seen that the verticals of the two images were not parallel, but crossed each other at small angle.

This my original diagram, however, is not well adapted to experiments on this subject for two reasons: 1. It is difficult to distinguish the image of one eye from that of the other. 2. It is difficult to control perfectly the convergence of the eyes. When the vertical lines approach each other, they, as it were, leap and cling together as a single line, even though they really cross at considerable angle; the really crossing lines, by a well known law of stereoscopic combination, being seen as a single line inclined to the visual plane. I therefore constructed a similar diagram, one-half of which consisted of black lines on a white ground and the other half of white lines on a black ground. It is convenient also to have two small circles, one on each half and similarly situated (fig. 6). If I place such a diagram perfectly perpendicularly before me, with the head perfectly erect and the eyes at precisely the same height as the small circles, and then stereoscopically combine the circles by crossing the eyes, I distinctly see the white and