ART. XVI.— On a new Theory of Light, proposed by John Smith, M.A.; by OGDEN N. ROOD, Professor of Chemistry in Troy University.

SEVERAL months ago, when attempting, by means of a revolving disc, to measure the time occupied by the explosion of small charges of gunpowder, the following observation was made: The flame of a burning-fluid lamp was viewed through a rotating disc provided with four radial slits, and it was found that a certain rapidity of rotation caused the lower part of the flame to assume a green hue, while by a diminished rate the whole flame was colored deep purplish red; a lower rate gave a violet tint, alternating with pure white.

It was evident that these appearances depended much on the state of the eye, for they often could be perceived only after it had become a little fatigued by the blinding effect consequent on the comparatively slow succession of the impressions of light; in addition to this, I found that my colleague, Prof. Vincent, though recognizing the green tint, was unable to perceive the red hue, with which my eyes at that very moment were dazzled.

I considered these appearances, therefore, as subjective, and laid the matter aside for future experiment. As, however, similar phenomena have been observed in England, and have been thought to have not only an objective existence, but to be worthy materials on which to build a new theory of light, it may not be amiss to enter into a slight examination of their nature.

In Mr. Smith's experiments, of which I have been able to obtain only the account given in the March number of this Journal, bright white light is allowed to act on the eye during a certain fraction of a second; it is succeeded by shadow or darkness, which lasts also during a certain short interval of time, when the operation is repeated anew, &c.

This pulsation of light and shade the author effects in a variety of ways: the result is color-a yellowish green, purple, pink, &c. Fechner, to whom we are indebted for extensive researches on sight, several years ago observed that white discs having black spiral figures painted on them, when set

in rotation exhibited colors which he considered subjective.*

That these colors are really subjective, the following simple experiment may serve to show: A blackened disc nine inches in diameter, was cut with four slits of the shape seen in the wood cut; the width of the slits at the circumference was of an inch; the disc was made to rotate before a bright cloud.

* Pogg. Ann. vol. xlv, p. 227.

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A rate of ten revolutions per second caused the cloud in a short time to appear of a deep red color, having in it a tinge of purple, or, according to Mr. Smith, the disc transmitted pink light; it was now viewed through a plate of orange-tinted glass. Previous experiment with a small telescope provided with a micrometer, and a flint glass prism, had shown that this orange colored glass readily transmitted the red, orange, yellow, and a portion of the green, but that it was opaque to the blue and violet rays; it was therefore fairly to be expected that if the disc was really transmitting red light, the plate of glass would do the same. The result was different; through the glass the disc appeared of a bright greenish-blue color. This experiment is very easy to make, and the effect is brilliant.

Plates of glass of other tints were now employed; the results are given below.

Medium.

Yellow glass,

Green

Red

Violet,

Tints of the Disc

Green, neutral, or faint red,
Red, neutral, or faint green.

As the green glass was nearly opaque to red rays, the effect of its use ought to have been darkness. It is evident by an inspection of this table that the disc really transmitted white, and not red light, which becoming colored by its passage through the plates of glass, induced in the retina, from time to time, the sensation of the complementary tint, more or less mingled with the original impression.

Having now shown that, contrary to Mr. Smith's supposition, the light transmitted by the revolving disc is really white, let us notice some of its effects on the eye.

For this purpose I caused perforated discs to revolve at uniform rates by means of clock-work; the arrangement being similar to that employed by Plateau. A blackened disc five inches in

diameter and perforated with four slits 7° 12′ in the width was set in rotation, and the bright sky viewed through it; the eye. of the observer being immediately behind the disc. With a rate of 11 revolutions per second, the appearance of the window was as in fig. 2; a central spot was colored bluish-green, the rest of the field was purple, or reddish purple, according to the state of the eye. The green spot remained always in the axis of vision, and moved with each change of it. With the exception of fluctuations in the outline of the spot, this appearance remained tolerably constant, as long as the rate of revolution continued the same. The spot or shadow was fringed with a narrow, faint blue border, indicated by the dotted line.

Upon increasing the rate of revolution, the bluishgreen spot expanded into an irregularly shaped ring, and continued to expand, filling the field, till the rate had become as high as 15 rotations per second, when often the field for an instant became of a greenish tint which was succeeded by a bluish tint; upon increasing the speed this also vanished. Still higher rates cease to produce any of these peculiar effects on the eye.

Upon slowly reducing the rate to 9 revolutions per second, the green spot contracted in dimensions, and assumed a yellow tint, while the field often became at the same time tinted deep crimson. With a rate still lower, the appearance of the field is variable and the tints flickering; it assumes sometimes a purple, a yellow, or a yellowish green tint.

This experiment I repeated a great number of times, with the same general result, and though it sometimes happened that the eye became insensible to these colors, from repetition, momentary rest in darkness restored this power for a short time. Thus it occurred that the tints were sometimes seen with great distinctness, while at others they could hardly be distinguished.

Upon a dark cloudy day to produce these effects it was found necessary to increase the width of the slits to 20°; from whence it was manifest that lack of intensity in the light might be made up by its longer duration.

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450

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750

575

It would appear then, from these experiments, that light from a bright cloud, if allowed to act on the eye repeatedly during from too of a second, developes subjective colors; that, however, the development of the subjective tint is dependent not so much on the length of time which the eye is exposed, as upon the interval of rest or shade which follows each exposure, may be shown in the following manner: In the experiment where with 11 revolutions a reddish purple was produced, the exposure lasted of a second; the interval of rest or shade was of a second; now a disc was cut similar to fig. 1, but having eight slits, each 7° 12′ in width, when it was found that 5.5 instead of 11.5 revolutions per second produced the purplishred tint; here the exposure was twice as long, but the interval of rest or shade nearly the same. With sixteen slits, 2 revolutions produced the same tint, the exposure being of course four times as long, but the interval of rest nearly the same. Determinations of the length of this interval are given below:Length of the intervals of shade required for the production of

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before these tints make their appearance, or how long the tints are actually seen by the eye, is another matter, and would no doubt require an elaborate series of experiments, though it would seem probable that at least half of the time of the above given intervals passes before the subjective color makes its appearance.

The table does not apply to the axial portion of the retina, which is almost always differently affected. That the change in susceptibility in the retina is progressive outwards, is shown by the gradual expansion of the green ring; that it varies from second to second, is seen in the fluctuations of the outline of this ring.

The occurrence and sequence of these subjective colors may easily be explained by supposing that during the interval of rest or shadow the action of the yellow rays diminishes more rapidly than that of the red, the red more rapidly again than that of the blue. If this takes place as indicated by the curves below, it is easy to understand the production of the tints, for if

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the moment after the blue has been developed white light be again presented to the eye, it is evident that neither purple nor red will be seen, sufficient time not having been allowed for their production.

The same reasoning applies to the axial portion of the retina, which, owing to its greater sensibility to such impressions of light, requires a somewhat longer interval of rest before the reaction occurs.

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If the impression be too strong, that is, if the light be too bright or the eye too long exposed to it, these peculiar effects are not observed, and during such short intervals of rest as ' or of a second the white clouds seen through the discs suffer no change in tint; but if a blackened disc twelve inches in diameter be cut as seen in the figure, with an aperture of 30°, and made to rotate before a white cloud at a rate of only one revolution per second, the eye placed as near it as possible, will most distinctly see, in the interval of darkness, an image of the sky, of a bluish-green tint. Whence it follows that an exposure of the eye to white light lasting of a second,

induces in it for a considerably longer time the sensation of this color.

AM. JOUR. SCI.-SECOND SERIES, VOL. XXX, No. 89.-SEPT., 1860.

In general these experiments seem to point out that after momentary exposure to white light subjective colors are induced in the eye, whose tint and duration are dependent on the strength of the impression received, as well as upon the length of time allowed for rest; these sensations of color apparently having a relation to the colors observed after looking at the sun, similar to that which a temporary disorder bears to a chronic affection. Troy University, March 20, 1860.

ART. XVII.—On the Meteor of November 15th, 1859; by Prof. H. A. NEWTON, Yale College.

In the forenoon of November 15th, 1859, about half-past nine o'clock, a meteor or meteorite was seen to descend towards the earth by many persons at widely different places. Throughout the southern part of New Jersey a tremendous explosion was heard immediately after its appearance. Mr. Benjamin V. Marsh of Philadelphia has published in the Journal of the Franklin Institute, a very valuable collection of statements of persons who saw it from various positions, from Newburyport, Mass., to Petersburg, Va. In the January and March numbers of this Journal Prof. Loomis has also given a selection from the accounts of several persons who were so fortunate as to witness it.

The physical circumstances attending its passage through the atmosphere, its explosion, and its entire or partial combustion are of interest and the accounts referred to are of great value in studying them. My present purpose, however, is to determine as accurately as possible the path and velocity of the meteorite. The result of my investigation has been to establish almost beyond a doubt the conclusion, that this body was not a member of the solar system but came to us from the stellar regions.

In several instances have the paths of meteors or shooting stars been determined and velocities been computed which would necessitate with respect to them the same conclusion. But in general the data are so vague that the result lacks the certainty which is afforded by the meteor of November. This is especially true of the velocities given by M. Petit of Toulouse. In not a single instance have the results given by him seemed to me worthy of confidence. I have thought, therefore, that a careful discussion of this meteor was desirable.

The observations from Alexandria, Va., and New Haven, Ct., seem for several reasons the best for determining its path through the atmosphere. Mr. Marsh quotes the following from a letter of Mr. Caleb S. Hallowell of Álexandria.

"Abram Martin, a student particularly well qualified for such observations, was fortunately, standing perfectly still with the