The instantaneous currents are distinguishable then, from those of a limited duration, inasmuch as they induce currents whose direction is contrary to their own, whilst that, for the others, the effects from the moment that their action commences are contrary to those which are of a continuous character.

That granted, when an inducing spiral acts on two spirals, A and B, it developes in each of them two currents of the second order and of the same direction; each of these latter create in the other a current of the third order, the direction of which is opposed to that of the current of the second order. We may conceive why it is that the quantity of electricity developed in each of the two spirals is smaller than if the other did not exist; but we may ask whether the current induced in each of the two spirals, in A, for instance, (the other being closed) is the difference between the current of the second order which determines the inducing spiral, and that of the third order which created the current of the second order of the spiral B. Though I am not yet in possession of the laws, following which these reactions are exercised, the very carefully made experiments which I report in detail in my memoir, establish, I think, in an incontestible manner, that the diminution of intensity produced in the current of each spiral by that of the second does not depend solely on the intensity of this latter one, and that it is necessary to regard some other circumstances, such as the relative position of the spirals with respect to the inductor.

Observations and Experiments on Light. By SAMUEL ADAMS, M.D., Professor of Chemistry and Natural History in Illenois College, Jacksonville, Ill.*

From Silliman's American Journal of Science.

SOMETIME in July, 1838, while on a mineralogical excursion, I accidentally noticed the wing-feather of a bird lying upon the ground; and being struck with the delicacy of its tints, I took it up to examine it. Observing that the vane of the feather appeared very

thin

• To the Editors of the American Journal of Science and Arts. Messrs. Editors-When "Observations and Experiments on Light" were forwarded to you for publication in the Journal of Science, I was not aware that Fraunhofer had anticipated the leading investigations of that communication. Pressing engagements, and frequent attacks of intermittent fever, prevented me from making so full an examination of the works of others on the subject as was desirable. I have since ascertained, that Fraunhofer has anticipated the leading results of my observations, in a series of experiments made by him by passing a beam of light through gratings, and examining the spectra produced through a telescope. (Herschel on Light, § 740, et seq.) I do not find, however, that the effect of the feather upon light has been before noticed, or that Fraunhofer ever exhibited the spectra upon a screen. You will oblige me by appending this as a note to my communication. Yours, &c.

Illinois College, May 21, 1841.

SAMUEL ADAMS.

and nearly transparent, I held it between my eye and the sky, which was very clear with the exception of a few fleecy clouds that contrasted finely with its rich blue. I was very much interested to observe that the clouds and all light coloured objects, which were highly illuminated, were seen through the vane of the feather beautifully fringed with the colours of the rainbow. I supposed that this phenomena depended upon the peculiar structure of the vane of the feather, and intended to investigate it as soon as I could find leisure. I did not however resume the subject till accident again called my attention to it.

About the 20th of June, 1839, while walking in the college grove, I happened to observe lying upon the ground some wing feathers of the jay, which reminded me of my former experiment. I collected the feathers, and after observing the same phenomena that I had noticed on the former occasion, I held the vane of the feather between my eye and the sun, and was greatly surprised at the gorgeous display of coloured spectra that were seen through it, arranged in the most exact mathematical order. The sun was seen in its natural position, slightly tinged with red, with its brightness considerably dimmed, and formed the intersecting point of two rows of coloured spectra, that crossed each other nearly at right angles. One of the rows of spectra formed a very acute angle with the shaft of the feather at its outer extremity, and the other was nearly at right angles with the shaft. In each coloured spectrum the side nearest to the sun was a mixture of violet and the contiguous rays of the prismatic spectrum, while the side furthest from the sun was uniformly red. The sun was slightly clouded when I made my first observations. Afterwards, when the sun shone perfectly clear, I observed that the angular spaces formed by the intersection of the two rows of coloured spectra were occupied by less brilliant spectra, arranged in the same order as the two rows above described.

On Monday, the 1st of July, 1839, I varied the experiments above described, by making my observations upon the flame of a lamp, instead of the sun. I found an advantage in this, as it enabled me to change the distance of the luminous object at pleasure. In looking through the vane of the wing feather of the wild pigeon at the flame of the lamp, I observed spectra coloured and arranged similarly to those which I saw when looking at the sun. I first looked at the lamp at the distance of eight or ten feet, and saw the two rows of coloured spectra above described entirely distinct from each other, with some faint appearances of spectra in the angular spaces near the lamp. As I approached the lamp (holding the feather to my eye and looking at the flame), the coloured spectra in the two rows gradually approximated to the flame of the lamp and to each other, their colours at the same time becoming less distinct and approaching to white light, while the spectra in the angular spaces became 'more perceptible. As I receded from the lamp, the spectra in the two rows receded from the central flame and from each other, their

Bb

colours at the same time becoming more distinct, and the spectra in the angular spaces gradually fading away.

My next step was, in connection with my colleague, Prof. Sturtevant, to introduce a small beam of light into a dark room by passing it through the vane of the wing feather of the jay. We observed coloured spectra arranged upon a screen in the manner described above. In the experiments which I first performed, the eye was the dark chamber and the retina the screen.

From reflecting upon these phenomena and conversing with Professor Sturtevant upon the subject, I was convinced that they were to be referred to difraction, produced by the passage of light through the minute foramina formed by the crossing and interlocking of the barbules of the feather. This conviction was strengthened by a microscopic examination of the vane of the feather, which exhibited an extremely minute lattice work between the barbs of the feather, formed by the crossing of the barbules, and by noticing that the lines in which the coloured spectra were arranged were perpendicular to the bars of the lattice. The similarity between the arrangement of the colours in the spectra upon the screen, and those of the external fringes produced by difraction, could not fail to be observed, and to incline me to the opinion that the law of interference, established by Dr. Young, had something to do with the production of the chromatic spectra. I was confirmed in this opinion by a series of experiments and measurements performed by Professor Sturtevant and myself, by which we ascertained that corresponding spectra received upon a screen at different distances from the feather, were not arranged in straight lines, but in curves. The curves seemed to belong to the hyperbola, and the latter to be formed by the section of a very acute cone. This is what might have been expected, as our experiments were performed upon parallel rays.

In order to understand the application of the law of interference* to the production of coloured spectra by the feather, it will be necessary to recur to the fundamental facts of difraction. Let it be borne in mind, that when a beam of light falls upon the edge of an opaque body, the rays which pass by the edge are divided into two portions, one of which is bent into the shadow of the opaque body, and the other is bent outward from the body. This separation of a beam of light into two parts is called difraction. For the sake of brevity and clearness I shall, in my subsequent remarks, speak of those rays which are bent into the shadow of the opaque body as inflected rays, and of those which are bent outward as deflected rays, and I shall use the terms inflection and deflection in strict accordance with these definitions. The plane of difraction is a plane passing through an inflected and a deflected ray which have diverged from the same point, and is always parallel to and passes through the unmodified beam of light. When the difracting edge is a straight line,

• See Interference, in Brewster's Optics, and Herschel on Light.

the plane of difraction is always perpendicular to a plane passing through the difracting edge and the corresponding outline of its shadow. In an irregular or curved difracting edge the same law will hold with regard to any indefinitely small portions of it, which may be assumed as straight lines.

I am aware that the terms inflection and difraction are used as synonymous by many who have written upon the subject of light. But without the definitions and limitations which I have just indicated, I should be compelled to resort to circumlocutions, which might render ambiguous the explanations which I am about to give of the phenomena of the feather. Again, I am not aware that the law which regulates the position of the plane of difraction has been stated by any other writer, although it is fairly inferrible from the facts which they have brought forward, as well as from experiments performed by myself, and which I hope to notice more fully in a subsequent communication. It will be seen in the sequel, that the law which regulates the position of the plane of difraction determines the angle which the two rows of coloured spectra make with each other.

Let us now turn our attention to the lattice-work formed by the crossing of the barbules of the feather, and inquire how the light passing through a single opening would be affected. The openings of the lattice are of course one of the four varieties of the parallelogram. The angles of these openings differ in the feathers of different birds, and in different feathers of the same bird. Let a b

b cd represent one of these openings; and let us suppose a beam of light passing through it perpendicular to the plane of the paper. It is evident that each of the sides of the opening will be a difracting edge; and if we take any two opposite sides ab, de, the inflected rays of one side will be bent in the same direction as the deflected rays of the other, and will be liable to interfere with each other, and produce coloured fringes upon a screen placed to receive the difracted light, and these fringes would extend on each side of the opening in a line perpendicular to the two sides in question. The same will be true of the other two sides, ad, be and thus we should have two rows of coloured fringes, whose lines of direction would be perpendicular respectively to the parallel sides of the opening, and consequently crossing each other at angles equal to those of the opening. But a part of the light would pass through the centre of the opening unbent, and would form upon the screen a white image at the intersecting point of the two rows of coloured fringes. Thus it will be seen, that a beam of light passing through a single opening of the kind above described, would be divided into nine parts, four being produced by the inflection of the four sides, four more by the deflection of the same, and one being the remains of the beams that pass on unmodified. Now let us suppose that a beam of light, instead of passing through a single opening, passes through an extremely minute lattice, containing an indefinite number

of such openings, as in the case of the feather. As all the bars of the lattice are parallel respectively to those which surround each individual opening, it is evident that the general effect upon the beam will be the same as that of a single opening, with this difference, that the range within which interference would take place, would be greatly enlarged, by enabling the inflected and deflected rays from different openings to interfere with each other; and thus the fringes, which are scarcely perceptible when formed by a single opening or a single edge, become brilliant spectra, when a beam of light is passed through a lattice of the kind described. All this is realised in the experiments with the feather. It is proper to remark, however, that the central white image is probably not formed entirely of unmodified light, but is partly produced by light slightly inflected by the opposite edges of the bars of the lattice, and corresponding with the internal fringes, first explained by Dr. Young upon the principle of interference. It is not improbable, that some of the deflected rays fall within the central white image and add to its brightness. The faint spectra in the angular spaces may be explained by supposing that they are formed by light which has undergone two inflections or two deflections, or one inflection and one deflection, by two contiguous bars of the lattice.

It should be noticed here, that all the coloured spectra, as well as the central white one, are considerably elongated in a direction perpendicular to the barbs of the feather. With a very delicate feather and a small luminous object, the eye can easily distinguish a row of coloured spectra arranged in the same direction. This is what might have been expected, and gives us some idea of the effect produced by passing a beam of light between extremely minute parallel bars arranged in the same plane very close to each other. The sun, moon, stars, the flame of a lamp, a small aperture in a dark room, &c., are convenient objects to be examined with the eye through the vane of a feather. When we wish to examine a luminous object through the vane of a feather, one of a dull or dark colour should be chosen, as a white feather transmits so much light as soon to exhaust the sensibility of the retina. For forming coloured spectra on a screen a white feather is preferable. Those feathers taken from the wing and tail, whose vanes approach the nearest to a plane, give the most regular arrangement of the spectra. The feathers of small birds, from the greater minuteness and delicacy of their structure, produce the most brilliant and extensive colours. We see here the same principle which Dr. Young applied to the construction of the Eriameter. In looking through the vane of a feather at a bright object, the most brilliant spectra are seen on the side towards the outer edge of the feather. This may be owing to the thinning out of the feather towards the edge.

If the above explanation of the phenomena of the feather be correct,

• See Brewster's Optics.