Phosphorus, the light-bearer, as its name implies, has the property, long supposed to be peculiar to it, of faintly shining in the dark. But, if a diamond is exposed to sunshine, and then withdrawn into darkness, it continues feebly luminous for a considerable time, and is, therefore, said to be phosphorescent. Other substances, as sulphuret of calcium, and sulphuret of barium, have also been long noted for this property, and recent researches have shown that, so far from being any thing peculiar, the same property is manifested in a much lower degree by a vast number of substances. The differences are in the time the phosphorescence continued after withdrawal from the sun's rays. It was found, in most instances, extremely short, only the small fraction of a second, and it became necessary to devise some means of measuring the time in different cases. A contrivance was necessary which should expose an object to the sun, and then jerk it quickly into total darkness, where it could be seen by the observer if it dragged any light along with it, for even the thousandth of a second.

A contrivance for this purpose was made by Edmund Becquerel, and called the phosphoroscope. It consisted of a train of wheels and pinions (Fig. 1) for producing rapid revolving motion. There was a

hollow barrel or case at the top of the machine, pierced with an opening, within which, as seen in the figure, the object to be experimented with is attached to a fixed stand. On the opposite side of the case there is another opening in a corresponding position, not shown in the figure. The outer case does not revolve, but within it there is a pair of disks (Fig. 2) rigidly connected upon a spindle which is turned by the machinery. Each of these disks has four openings, those of the one being not opposite, but midway between those of the other. Of course, then, when these disks are inside the case, it is impossible to see through. The arrangement is then set up in the window of a darkened room, so that one side is turned toward the sun, and the

other toward the observer; and, when the disks are turned, the object is alternately exposed to the light from one side, and to the eye from the other; that is, it is seen in a moment after exposure to light, and the duration of the moment can be determined by the rapidity of the rotation. The object, therefore, if not phosphorescent, will never be seen by the observer, as it is always in darkness, except when it is hidden by the intervening disk. But, if its phosphorescence lasts as long as an eighth part of the time of one rotation, it will become visible in the darkness. Suppose, now, that the disks are made to revolve a hundred times in a second, and that the body observed is visible, it is then proved that its phosphorescence lasts the one eight-hundredth of a second, that being the time which elapses between its exposure to the sun and its exposure to the eye. When examined in this way, a very large number of bodies show traces of phosphorescence, although in some cases it is found to last no longer than the ten-thousandth part of a second.

The question was thus opened whether phosphorescence is not a general property of matter, and, to determine this, with the conditions of its manifestation, a more thorough investigation of the subject was needed. Prof. Rood proposed to undertake it, using, if possible, an instantaneous source of illumination--the electric spark. But, in en

tering upon the inquiry, he soon found himself involved in preliminary difficulties with the spark itself. His phosphorescent investigations remain yet to be carried out, but the results obtained relative to the electric flash are of extreme interest. The full account of the research is given in a series of papers published in Silliman's Journal, and, if the reader finds the following statement insufficient in its details, he will know where to go for further explanations.

Since the time of Franklin, the lightning-flash has been regarded as a gigantic electric spark produced in the atmosphere; the inquiry, therefore, involved the nature of the meteorological discharge, as well as of the spark artificially produced. Various attempts to determine the duration of lightning have been made, with varying results. Faraday observed it, without any instruments for measuring the time, which seemed to last for a second, but he was doubtful if part of the effect was not due to the lingering phosphorescence of the cloud. Decharme observed the lightning-flashes from a distant storm, which also appeared to last for from a half to an entire second. Prof. Dove employed a revolving disk with colored sectors, and satisfied himself that single flashes of lightning often consisted of a number of instantaneous discharges. It is well known that, when a rapidly-moving train of cars is illuminated at night by lightning, it seems to stand still, that is, the duration of the flash is so brief that no motion of the train is perceptible while it lasts. The wheels are sharply defined as if perfectly motionless, but if they had a blurred aspect we should know that the illumination lasted sufficiently long to render the motion perceptible. Prof. Rood extemporized a simple contrivance for observing lightning, which acted upon this principle. It consisted of a white card-board disk, five inches in diameter, with a steel shawl-pin for an axis, on which it was made to revolve by striking the edge. He traced black figures near the circumference of the disk, and when it was in rapid motion these figures were sometimes seen as sharply as though they had been stationary, although they were often blurred as though the disk had moved through a few degrees during the act of discharge. He then cut narrow, radial apertures into the circumference of the disk, and observed the lightning through these openings. Here, again, the apertures were sometimes seen quite unchanged, but they were more frequently elongated into well-defined streaks some degrees in length. He afterward measured the average rate of rotation imparted to the disk in this way, and arrived at the conclusion that the lightning-flashes on the occasion referred to had a duration of about one five-hundredth cf a second. Dissatisfied with the roughness of these observations, Prof. Rood arranged a small train of toothed wheels driven by a spring, which rotated a circular pasteboard disk with four open sectors. This instrument gave more regular and precise results; and, while it was shown that the flash sometimes lasts for a whole second, the suggestion of Dove was clearly verified that each flash "consisted of a consider

able number of isolated and apparently instantaneous electrical discharges, the interval between the components being so small that, to the naked eye, they constituted a continuous act."

Several curious effects were observed in these experiments. Working with a disk having a single narrow opening, the multiple elements of the discharge were detected with great regularity, and Prof. Rood several times, instead of seeing the opening single, noticed that it had a form resembling the letter X or V, the lines in different positions of the disk having, as it were, got crossed in his eyes by their quick changes of position. On several occasions, when observing with the naked eye, the normal zigzag flashes lasted not less than a second, and the light seemed to pour steadily in a stream from the cloud to the earth. Observations made in the area occupied by a storm, out beyond its edge, and when it was quite distant, gave results that were identical, which the professor thinks furnishes an "argument in support of the hypothesis that zigzag lightning, heat and sheet lightning, etc., are really identical, being, in point of fact, due to the same cause but viewed under different conditions." As the result of these experiments, Prof. Rood concludes: "It is evident, from the foregoing, that the nature of the lightning-discharge is more complicated than has been generally supposed; it is usually, if not always, multiple in character, and the duration of the isolated constituents varies very much, ranging from intervals of time shorter than one one-thousandth of a second up to others at least as great as one-twentieth of a second; and, furthermore, what is singular, a variety of this kind may sometimes be found in the components of a single flash."

Such being the rough conclusions reached concerning the duration of the spark upon a grand scale, let us now consider the results of experiment upon it where all the conditions are in command. In 1835, Mr. Wheatstone attempted to measure the spark of a Leyden jar charged by a common frictional machine. The light from the spark was received upon a mirror mounted upon an axle capable of a high rate of revolution. The image of the spark, being thrown upon the mirror, was reflected to a distant point, and the time of the spark was inferred from the fixity or movement of the image. By using this arrangement, Mr. Wheatstone concluded that the discharge may take place within the millionth of a second; a result which was accepted by the scientific world for a quarter of a century. In 1858, a German named Feddersen, an accomplished physicist, dissatisfied with Wheatstone's results, entered upon a careful reëxamination of the subject. He used the revolving-mirror arrangement with frictional electricity; but, as Wheatstone had driven his machinery by strings, Feddersen adopted a train of toothed wheels, and with this form of mechanism he found that the image of the spark was drawn out by the revolving mirror into a whitish streak which indicated that the time of the discharge was not less than the twenty-five-thousandth of a second, while

it was inferred that the spark, instead of being a simple effect, is composite like the lightning, and is made up of several elements.

Such were the incomplete and discordant results of the investigation when it was undertaken by Prof. Rood. The arrangement he devised consisted of two parts, one for the production of the spark, and the other for measuring it. Fig. 3 represents the first combination. A galvanic battery was used to generate the electricity; this was connected with a large Ruhmkorff coil, which was again connected with a Leyden jar, and this with the electrodes for producing the spark, S,

FIG. 3.

Galvanic Battery.

Ruhmkorff Induction-Coil.

Leyden Jar.

Electrodes and Spark.

which were adjustable for varying its "striking distance." Connected with the wires between the battery and the coil was an automatic "interruptor" for breaking the circuit from three to six times in a second, by which the frequency of the discharges could be regulated. Leyden jars of different sizes could be used so as to give sparks of all degrees of strength and intensity.

In the second part of his arrangement, Prof. Rood, like his predecessors, employed a revolving mirror, turned by the gearing of Becquerel's phosphoroscope (Fig. 1), with the addition of an extra wheel and a weight to drive it. With this he could get 350 revolutions of the mirror per second, with a smooth and uniform motion. In order to measure exactly the rate of rotation, the cylinder on the lowest wheel was made to wind up a fillet of paper, upon which dots were made by an electro-magnetic apparatus, regulated by a seconds-pendulum, when a simple calculation gave the rate of the wheel to which the mirror was attached, and the regularity of the train was thus put to a sharp test. The light of the spark S (Fig. 4), passing through an achromatic lens, 7, struck the mirror, m, and was reflected upward, forming an image at i, on the plate of ground glass G. The image of the spark on the ground glass was viewed from above, and its position and form were carefully measured by several methods. Of course, if the