tive, merely by its position, of a special vision by that eye, while from the conditions of the experiment these circles are in fact reversed in their places as compared with the tubes and eyes to which they appertain.

We have seen in the above experiments that when an object is presented to one eye without any accompanying circumstances leading us to refer the visual act specially to this or to the other eye we have a consciousness of seeing it equally with both eyes. The same result occurs when separate objects are presented to the two eyes, provided as before, extraneous sources of suggestion are excluded.

Exp. 5. Thus if we place on the black slide of the stereoscope two spots, differing either in shape or color, one before each eye, we perceive them both in the middle or binocular direction, each seemingly visible in an equal degree to both eyes, the one being seen through or upon the other according to the fitful attention or suggestion of the moment. A pleasing modification of this experiment is made by using two unequal white spots on the black slide and interposing a green or other colored glass between one of them and the lens. The spot which appears colored will give as strongly the impression of being seen by both eyes as the white one, in spite of our knowledge of the position of the colored glass.

Even in cases where the two objects are wholly unlike, and at very different distances from the eyes by which they are severally regarded, this feeling of a common or united visual act in regard to each of them is often easily recognized. Of this we have a ready illustration in the familiar experiment on ocular parallax in which a distant object, hidden from one eye by an interposed finger or pencil, is seen through or behind the pencil when both eyes are directed towards the distant object.

Exp. 6. To observe this effect satisfactorily it is well to make the experiment in an apartment in which a single small lamp is placed at some distance from the spot on which we stand. Looking intently at the lamp, we bring the pencil before the face in such position as to give us an image on each side of the lamp, and then move the pencil toward the right until its left hand image seems to coincide in direction and position with the lamp, which appears to shine through or to partially replace it. As we continue to look thus at the lamp, we have a clear impression that both lamp and pencil are equally visible to both eyes, and without some consideration of the previous adjustment and motions we are unable to determine which is actually visible to the right and which to the left eye.

The same experiment furnishes also an incidental illustration of the principle of transposed visual reference before alluded to.

If, while the above adjustment is maintained, we contemplate the other image of the pencil situated some distance to the right of the lamp, and endeavor to decide, from the mere visual impression, to which eye it appertains, we almost unfailingly refer it to the right eye as that which most nearly fronts it, although obviously it belongs to the other, as will be found at once on closing either eye.

Where the eyes are externally very sensitive, any strong illumination of one as compared with the other will interfere with the effect above described by referring the impression specially to the eye thus unduly excited. In snch cases the observation is best made in a moderately lighted room by interposing the pencil between the eye and a vertical stripe on the wall.

Exp. 7. Recurring to Exp. 2, in which with a tube in front of one eye we perceive a bright circle on the wall in the medial direction, we may obtain a pleasing illustration of the point now under consideration by bringing a dark card or book or even the hand between the uncovered eye and the wall. The spot instead of being intercepted will appear as a perforation in the opaque screen.

Here as in the case of the pencil and lamp, the bright circle and the screen are both optically referred to the intersection of the two lines of view. But the luminous circle almost or entirely obliterates the corresponding part of the screen. As the full view of the screen and its connections continually remind us that it is in front of the uncovered eye, we are led to refer the luminous circle seen as coincident with a part of it, to the same eye, and thus to believe that we are looking through the screen with that eye. It is however not difficult, by intently regarding the luminous circle, so to counteract the force of this extraneous suggestion as to feel even in this case as if the circle were equally in view to both eyes.

These considerations explain very simply the experiment of the pseudo-diascope described by Mr. Ward of Manchester, which like several of those above mentioned is but an instance of the old observation of Da Vinci, that when we see behind a small opaque object presented near the eyes "it becomes as it were transparent." In making this experiment with a tube of paper supported between the thumb and forefinger of the left hand and held before the right eye so that the back of the hand may be some inches in advance of the left eye, it will be noticed that the effect varies with the amount of convergence of the eyes and that the bright perforation in the hand may or may not be referred to the left eye according to the force of the accessory suggestions or the intentness with which we fix our gaze upon the distant spot to which the axes are converged.

In concluding, it may be remarked that the experiments which have been described are for the most part too obvious and familiar to have merited such a special notice but for the peculiar and in some respects new interpretation which they have offered of many visual phenomena. Considered in this relation we are I think entitled to conclude from them :—

First that the retinal impression of an object presented directly to either eye is accompanied by the feeling of a united visual act, and of itself gives no indication of the particular eye impressed; and:

Second, that the reference of the impression to one eye rather than the other is the result of collateral suggestion, which may either locate the image in the eye that actually receives it, or may transpose it seemingly to the other, according to the particular conditions of the observation.

ART. XLI.-Correspondence of J. Nicklès, of Nancy, France.

Physical Chemistry.-Polarized Light employed as a Reagent.-BIOT, who entered his 86th year on the 21st of April last, and who continues to labor with the ardor of youth, has just published a résume of his complete works on circular polarization, applied to the study of chemical species. These researches, which he has greatly promoted, have often occupied us and daily gained more importance. Biot has given us the following account of the manner in which he was led to the discovery of the fundamental fact which has given point to all his labors in this field. He says:

Accident, that great promoter of physical novelties, gave me in 1815 the first hint in some experiments made for an entirely different object.

Wishing to study the changes of a polarized ray, when it passes very obliquely through thin plates of sulphate of lime, I had enclosed such a plate in a large metallic tube, about 8 centimetres long terminated by parallel faces of glass, and filled with colorless essence of turpentine. This was attached to a metallic rod projecting out transversely, which turned on its axis around the centre of a graduated circle, allowing me to change at pleasure the inclination of the interior incidence of the polarized ray. Having thus brought the plane of the plate in the direction of the ray itself, so as to fix the point of departure of their inclinations, the principal section of the analyzing prism being previously placed in the plane of the primitive polarization so as not to disturb it, I perceived in that part of the field of view occupied by the essence an indistinct, extraordinary image, of a blue color, caused by the interposed

essence.

Water and alcohol previously introduced into the same tube, produced nothing similar. The effect observed, was not therefore due to the interposition of the essence, as a liquid mass. It must necessarily result from some physical property, peculiar to the substance which produced it.

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

I hence inferred that there was a deviation from the plane of polarization, to the right or the left, as in plates of quartz cut perpendicular to the axis. Revolving the analyzing prism successively in these two directions, towards the left, the blue image gradually grew fainter, until at a few degrees distance, it disappeared entirely, and reappeared beyond colored red, which I had found to be a distinctive character of rotary phenomena produced by plates of left-handed quartz. Hence I immediately concluded that a phenomenon of the same kind, was here presented except that it was produced by the successive actions of similar particles of the

essence."

The principal fact, its molecular character, and the general consequences which are deduced from it, were made known to the Academy of Sciences, the 23d of October, 1819, and were published in a few days after in the Bulletin of the Philomathic Society of the same year, p. 190.

Since then Biot has most perseveringly prosecuted these general studies and labored to have chemists adopt them as a means of studying the intimate constitution of bodies without decomposing them, inasmuch as chemical analysis is applied to the study of those bodies only after they have ceased to exist: he explains the difficulties he has surmounted in persuading chemists to adopt this method. It is credible that these difficulties would not have been overcome, if the author of circular polarization had been an obscure savan-just as the value of the beautiful discoveries of Pasteur, would not yet have been recognized had not Biot been their interpreter. Biot closed his recapitulation of the principal discoveries of which the circular rotatory power of molecules had been alike the occasion and the instrument by the reflection, if his investigations had left undiscovered other facts of a kindred nature, no one could regret the involuntary omission more than himself.

On the existence of new simple bodies.-The beautiful labors which Bunsen and Kirchhoff have recently published (see page 415) on this interesting topic, the great advantage which they derive from the wonderful and peculiar action of the different metals upon the solar spectrum, the use made of it for qualitative chemical analysis and which they propose to make of it, in investigating the nature of the light of the sun and the fixed stars, all these and many other questions raised by their memoir, have recalled attention to a research published ten years ago by Leon Foucault on the rays of the electrical spectrum. This physicist first proposed the use of points of gas carbon to form the voltaic arc.

*

In studying this arc in 1847-48, he discovered that the ray D of the electrical spectrum coincides with that of the solar spectrum, and hence we may produce a superposition of two spectra, by throwing on the voltaic arc a solar image formed by a convex lens. When metals, which produce the ray D only feebly, as iron or copper, are used as poles, remarkable intensity can be secured, by touching them with a salt of potash, soda, or lime.

Foucault further adds :-"Before inferring anything from the constant presence of the ray D we must ascertain if its appearance does not indicate the same material mingled with all our conductors. Still this phenom

* L'Institut, Feb’y, 1849; see also this Journal, [2], xxix, 424.

enon urges us to study the spectra of the stars-for if fortunately the same ray were there found, stellary astronomy would be advanced."

We recall in fact, that relying on their own observations, Messrs. B. and K. have admitted the presence of potassa and soda in the sunbeam, (Journal de Pharmacie et de Chimie, May, 1860, and this Journal, xxix, p. 424.) in the complete account they lately gave in Poggendorff's Annals, T. cxl, p. 161. These savans have shown how sensitive this characteristic is that even infinitesimal traces of potassa, soda and lithia can be recognized in the rays they produce and these characteristics continue even when the bases are reunited.

We will not dwell further upon this important research which is elsewhere presented in this Journal: but we should point out another direction of these researches, that which relates to the discovery of new simple bodies. Messrs. Bunsen and Kirchhoff made known lately the probable existence of a fourth alkaline metal, placed in relation to the rays of its spectrum immediately after lithium and preceding strontium.

It is thought that this process will determine the existence of the new metal dianium announced by M. Kobell (this volume, July, 1860, p. 123) and whose reality is contested by H. Rose.

Catalysis and Contact Actions.-The class of phenomena in chemicophysical research which for the want of a better term we distinguish as above, has been enriched by the discoveries of H. Loewel,* whose researches have remained hitherto too little known, but which demand attention from the very original facts they have brought to light; which are the fruits of several years of observation.

His researches on supersaturated saline solutions.—It is well known that a supersaturated solution of crystallized sulphate of soda exposed to the air crystallizes suddenly when touched by a glass rod, but that it does not crystallize when this rod is heated to 100° C. This fact is connected with many investigations which Loewel followed for a part of his life, and which are the subject of a critical analysis published by Hirn, a pupil of Loewel, who is well known by his interesting re

searches on heat.

Hirn's analysis of his teacher's results has rather the character of a dissertation filled with new facts, which although they may have been disputed, have yet remained so hidden in the diffuse memoirs of Loewel that they appear, as it were, now for the first time. As the research is too long to reproduce here, we content ourselves with citations from it.

* Poggendorff's Biographical Dictionary speaks of this Author thus: "Loewel, Henri, Director of the chemical works at Choisy-Le-Roi, from 1815 to 1819, since then colorist at Münster (Upper Rhine), born on the 29th of Sept. 1795 at Münster, died 5th of Sept. 1856, at Colmar: his published researches are:

Upon some salts of chromium, Annales de Chim, et de Phys. (Ser. iii, T. xiv, 1845.) Upon the chlorids of chromium (ib. xv, 1845.)

On the supersaturation of saline solutions (ib., xxix, 1850, xxxiii, 1851, xxxvii, 1853, xliii, 1855, xliv, 1855, xlix, 1857.)

Upon the action of zinc and iron upon the solution of the salts of sesquioxyd of chromium (ib., xl, 1854).

On the solubility of the carbonate of soda (ib. xliv, 1855).

On the chlorohydrate and sulphate of chromium (Comptes Rendus, xx, 1845.) See also Le Recueil du Journal de Pharmacie et de Chimie, which has also published these memoirs, a little earlier than the dates above quoted,”