houses above and nearer, and the larger farther away in the distance.

On combining in a similar manner one of those skeleton polyhedra so frequently used to illustrate stereoscopic principles, I find the stereoscopic effect equally perfect as with the stereoscope except that the nearer triangular face is smaller than the farther one instead of the contrary. Nothing can exhibit more clearly than these experiments, the entire distinctness of the binocular from every other kind of perspective.

Stereoscopic pictures may be combined with the naked eye, also beyond the plane of the card in the manner of a stereoscope; but there are two difficulties in the way of success in this kind of combination. In the first place in most stereoscopic pictures, identical points are farther apart than the eyes, and therefore, cannot be combined beyond the pictures without the aid of lenses or prisms. In the second place, even if the pictures are not farther apart than the eyes, and may therefore be thus combined, the dissociation of the focal from the axial adjustment, as already explained in my first paper* is difficult and imperfect, and the combined picture therefore is not clear.

I wish now to apply the method proposed in my last article, in the representation of stereoscopic phenomena. The usual method, which I have used in figs. 1, 2, 3, and 4, because it is familiar, represents perfectly the position of objects seen single and therefore their relative distance or the depth of space, when the eyes are directed upon them consecutively; but cannot represent the position of double images in the stereoscope any more than it can in natural vision. Fig. 7, gives the mode of representing by the usual method. AR, AL is the position of the optic axes when objects a a in the foregrounds are combined at A and bb't the position of the double images of bb, seen at the same distance as A; BR, BL the direction of the optic axes when objects bb in the backgrounds are combined and seen at B and aa', the apparent position of aa at the same distance as B. Fig. 8 gives the same when pictures are combined by squinting.

Now it is evident that this mode of representation is not true, for we do not refer bb' to the same distance as A, when we look at A, nor aa' to the same distance as B when we look at B. The whole stereoscopic effect would be lost if we did. On the other hand my method of representation gives the true apparent positions of the double images as we now proceed to show.

When we gaze through a stereoscope the two pictures seem to slide inward over each other until they unite to form a single *This Jour., II, xlvii, pp. 73 and 76.

As in my previous article dashed letters mean left-eye images.

picture in the middle. The septum of the stereoscope is therefore doubled heteronymously, and forms two parallel planes or walls bounding the field of view on either side. Between these two bounding planes, the eye (combined eyes) from its apparent central position seems to look straight forward upon the scene. As soon as we converge the eyes upon any object in the scene, the two septa or bounding planes seem to converge to the same degree, and if produced would meet at the point of sight, if this be in the median line, or in any case at the distance of the point of sight. As the point of sight however, is always far beyond the septa, practically the septa will seem nearly or quite parallel. Fig. 9 gives the actual relation of parts and the position of the visual lines when bb objects in the backgrounds. of the pictures are united and seen at B; fig. 10 gives the visual result. The two pictures are slid over each other each a half interocular space until bb fig. 9, unites in the middle b fig. 10, and aa slide by each other and become heteronymous. The combined picture b is seen at B (fig. 10), because we are conscious of an optic convergence suitable for that distance, or in other words we are conscious of looking at that distance. In this mode of representation the position of B, when it is on the median line, is determined by the intersection of the double septa or median lines n S, n'S" produced; and the position of aa' of the scene is determined by the intersection of the lines from the eye through aa of the card (ray lines), with the median lines n S, n'S". It will be observed that these double images occupy precisely the position of those of an object at A fig. 9. Fig. 11, gives the relation of parts and the direction of the optic axes, when objects aa in the foregrounds of the pictures are combined and seen at A, and fig. 12, the visual result. To combine aa (fig. 11), bb do not slide by each other, and are therefore homonymous, and are therefore referred beyond A and seen at bb' (fig. 12) precisely as if they were the double images of an object situated at B (fig. 11). In the representation (fig. 12) the exact position of bb' of the scene, is determined as before by the intersection of median lines with the ray lines.

The phenomena of combination by squinting is represented in figs. 13, 14, 15, of which fig. 13 represents the actual relation of parts, and the direction of the optic axes when foregrounds and backgrounds of the pictures r and are consecutively combined. In this case the mounting is supposed to be changed so as to make the perspective natural. Fig. 14 represents the visual result when the foregrounds are united, and fig. 15, when the backgrounds are united. The positions of the double images in the scene are determined as before. In the case of combination by squinting, the two images of the card do not slide over each other inward, as in the stereoscope, but

outward; so that, as already stated, the right-eye image of the left picture covers the left-eye image of the right picture, to form the binocular picture or scene; while homonymous images of the right and left pictures are seen to the right and left. I have represented this in figs. 14 and 15, where r and are right and left pictures as seen by the right eye, and r'' the same as seen by the left eye. By careful inspection, after what I have already said, the figures will explain themselves. It is true, this mode of representation is complex, and, for those unaccustomed to binocular experiments, perhaps difficult to understand; but it has the advantage of truly representing the some. what complex visual phenomena.

Oakland, Cal., March 20, 1871.

ART. II. On three Masses of Meteoric Iron, from Augusta Co., Virginia; by J. W. MALLET, Professor of Anal. and Applied Chemistry, University of Virginia.

NEARLY two years ago I learned that a lump of iron, which from the description given of it I supposed to be meteoric, had been turned up by the plough in Augusta Co. in this State, and soon afterwards I obtained possession of this specimen by the kind assistance of Hon. J. B. Baldwin of Staunton. It proved to be beyond question a meteorite, weighing about 56 lbs.

A few months later, I saw at the Annual Fair of the State Agricultural Society in Richmond, a second mass, of smaller size, weighing about 36 lbs., which had come from the same county, and was exhibited along with some iron ores by Maj. Jed. Hotchkiss of Staunton. Learning from me that I was about to examine and analyze my own specimen, and was anxious to compare it with the other found in the same part of the country, Maj. Hotchkiss was obliging enough to lend me the latter, and to permit me to cut off enough for analysis. Quite recently he has placed in my hands a third specimenalso from Augusta Co.-weighing but about 34 lbs.

I shall speak of these three masses as No. 1, No. 2, and No. 3, in the order in which they are mentioned above; No. 1 being my own specimen, and Nos. 2 and 3 those of Maj. Hotchkiss.

All three present quite the same general appearance. They are of a very irregular pear shape, one end of each mass being larger and more rounded than the other-the smaller end of each is somewhat flattened, but by concave surfaces, in one direction. No. 1 was more massive and rounded than the

others-No. 2 most flattened-the latter had some rude resemblance in shape to a shoulder of mutton. The dimensions of the masses before cutting were as follows:

A pretty good idea of the shape and size may be obtained from the accompanying figures, from photographs of the original specimens with attached scale. The exact weights before cutting were,

No. 1.

25,429 grams.

No. 2.

16,441 grams.

No. 3.

No. 3.

1,644 grams.

the masses being entire, nothing having been previously detached from any one of them.

The surface of each of the masses is rough and irregular. At some points, which have been rubbed, the iron exhibits its metallic luster, and traces of its crystalline character may be observed, but nearly the whole surface is covered with a dark brown crust, consisting essentially of hydrated ferric oxide, which varies from about an eighth to a third of an inch in thickness. This crust is hard, and pretty firmly adherent. On exposure to moist air a rusty liquid exudes in drops from numerous points upon the surface, and in this watery liquid chlorine, iron (chiefly as ferrous chloride), and nickel were detected. The masses are of course magnetic, and on examination give evidence of feeble magnetic polarity, with multiple poles.

The union of hardness and toughness in the iron makes it quite difficult to cut, and in attempting to obtain with the planing machine a slice of considerable size the ordinary cutting tools were blunted and broken; it was found necessary to drill a row of holes and connect these by a cut made with the planer.