spark was absolutely instantaneous, its image thrown upon the ground glass would be exactly the same, whether the mirror was motionless or was revolving at the highest speed. But, if the spark had an appreciable duration, its image would be prolonged or drawn out into a streak, the length of which must depend upon the time of discharge. The rate of the mirror's rotation being known, also the distance, m i, and the length of the streak, it was easy to calculate the total duration of the spark.

Prof. Rood now had the subtile agent he was pursuing pretty effectually in his grasp, and the results that came out were very striking. The ordinary spark was found to be a highly-complex effect; to consist of diverse and successive elements, and, in fact, to have its periods and orderly history just like the geology of the globe. But, while the "vast durations" of Lyell and Dana are vague and inferential, these infinitesimal periods could be demonstrated with the greatest exactness. The previous discordant results were reconciled, Feddersen being justified in assigning a longer period for the total duration of the spark, and Wheatstone's time holding true of its elements.

With a Leyden jar of about a quart capacity (114.4 square inches of coating), and all the connections as short as possible, so as to offer the least amount of resistance to the electric flow, with brass balls as electrodes, with a striking distance of about the twenty-fifth of an inch, and the velocity of the mirror up to 223 per second, the image of the spark thrown upon the ground glass and viewed by the naked eye was drawn out into a streak one and a half or two inches long, the length, however, varying with the speed of the mirror. The aspects of the image are represented in Fig. 5. The first part was pure white, which shaded into a brownish-yellow tint, passing on into a pretty distinct green. When a polished plate of glass was substituted for the ground glass, and a small magnitier was used to observe the image, a series of bright points, on each side of the streak, became visible, in the positions indicated by the dots in Fig. 5. With high velocities, this succession of points was beautifully developed, and it consists of a series of separate discharges following the first. It was thus found that the

Leyden jar furnished a number of single sparks, each time the coil was excited, the number varying between one and thirty, according to circumstances. The whole proceeding consumed an interval of time often as great as one-fiftieth of a second; that is, the jar loaded up and discharged itself twenty or thirty times in that period. Prof. Rood found the number of elements of the spark to vary with its length, the nature of the electrodes, and the size of the jar. Short sparks are more complex than long ones, small jars give more than large ones, and metallic points a greater number than balls. The point to be determined was, the duration of the several elements of the spark, and especially of its quickest element. In one case of a discharge lasting the fiftieth of a second, it began with an ordinary spark, followed by a pale-violet light, lasting about one-sixtieth of a second, and then came a compact

body of ten or twenty sparks, this last act continuing for about one two-hundredth of a second. The results of the inquiry are thus stated by Prof. Rood: "From the foregoing, then, it appears that, if a jar, having a metallic coating of about one hundred square inches, be connected, as above described, with an induction-coil, its discharge will be effected by a considerable number of acts, of which the first is by far the most intense. Further, the metallic particles, heated up by the first discharge to a white heat, almost instantly assume a lower temperature, marked by a corresponding change from white to brownish yellow; and, as their temperature continues to fall, the tint changes, in the case of brass electrodes, to green; in that of platinum, to a gray or violet-gray. These observations further demonstrated the fact that four ten-millionths of a second is an interval of time quite sufficient for the production of distinct vision."

It was also shown that the first act of the electric explosion, represented by the white band, lasted through an interval of time so short as to be immeasurable. It was proved that it could not occupy more

than the millionth or the half a millionth of a second, but how much less time it might occupy remained to be determined.

Prof. Rood now prepared for a more rigorous course of experiments. He used a small Leyden jar, with a surface of eleven inches, about equal to a moderate-sized wine-glass. To secure greater exactness of observation, he devised a peculiar micrometer, consisting of five lines ruled on a plate of glass smoked by lamp-black. This plate was placed between 1 and S (see Fig. 4), but quite near to the latter, and an image of the lines reflected from the mirror was formed on the clear glass at i. The lines were observed by a microscope magnifying ten diameters. In using this micrometer, the measurement was effected by noticing at what rate of the revolving mirror the lines in the image at i were obliterated, this obliteration being due to the circumstance that by the motion of the mirror the dark lines were superposed on the bright lines. The individual spark now produced was about a millionth of a second in duration, but the faint train was still observable. There was still the brilliant body of the spark appearing, first followed by a faint streak of less than one-hundredth the illuminating power of the first stage. The diagram, Fig. 6, represents the intensity

FIG. 6.

Intensity.

Duration.

and time of the spark. The elevation, or peak, a b, shows the intensity of the first compact body of the spark, and the line a c the duration of the whole effect. The point was to get the time of a b, which Prof. Rood had proved must be regarded as a distinct act in the succession of effects. All precautions for observation being carefully made, the driving-weight was gradually increased, and the speed of the mirror carried up to 350 revolutions per second, when the lines of the image, which at first remained visibly as distinct as with a stationary mirror, became regularly less distinct, and at length vanished by the gradual superposition of the white and black lines. Prof. Rood says: "It was proved successively that the duration was less than eighty, sixty-eight, fifty-nine, fifty-five billionths of a second; and, finally, the lines, after growing fainter and fainter, entirely disappeared, giving as the result a duration of forty-eight billionths of a second." By reducing the striking distance, a still lower figure was

reached, so that the professor states that "the duration of the first act of the electrical discharge is in certain cases only forty billionths of a second, an interval of time just sufficient to enable a ray of light to travel over forty feet." The duration was twenty-five times smaller than had ever before been measured. In this infinitesimal portion of time a strong and distinct impression upon the retina is made, so that "the letters on a printed page are plainly to be seen; also, if a polariscope be used, the cross and rings around the axis of crystals can be observed with all their peculiarities." Nor is this all; "as the obliteration of the micrometric lines could only take place from the circumstance that the retina retains and combines a whole series of impressions whose joint duration is forty billionths of a second, it follows that a much smaller interval of time will suffice for vision. If we limit the number of views of the lines presented to the eye in a single. case to ten, it would result that four billionths of a second is sufficient for human vision."

We saw at the outset how much an act of vision involves, and we have now some idea of how long it takes. If the discharge of the thunder-cloud occupies, as was stated, the one five-hundredth of a second, the "interviews" of our philosopher with the "amber-spirit' were at least fifty thousand times "quicker than lightning."

THE EMOTIONAL LANGUAGE OF THE FUTURE.

MR.

R. SPENCER recently called the attention, in a very interesting passage of his "Psychology," to those secondary signs of a feeling which are to be found in abortive attempts to conceal it. "A state of mauvaise honte," he well says, "otherwise tolerably well concealed, is indicated by an obvious difficulty in finding fit positions for the hands." A great mental agitation, though prevented from breaking out into violent expression, is pretty certain to betray itself in the awkward, shuffling movements which are made to curb and suppress it. Such indirect signs of emotion Mr. Spencer calls its secondary natural language.

The fact that many of our emotions now betray themselves only through the incompleteness of the effort of will to disguise them is not a little curious, and offers several lines of interesting inquiry. It at once suggests how very little play for emotional expression the conditions of modern society appear to allow. For it seems tolerably certain that the voluntary hiding of feeling is a late attainment in human development, and is forced on us simply by the needs of advancing civilization. Savages, for the most part, know little of concealing their passions, and this makes them so good a psychological

study. Children, too, who may be supposed to represent the earlier acquirements of the race, are proverbially unfettered in the expression of their sentiments. In like manner, in the various ranks of our civilized society, we see that, while a cultivated lady appears to all distant onlookers to have a mind dispassionate and undisturbed by agitating feelings, a west-country maid reveals her curiosity and wonder, her alternations of joy and misery, with scarcely a trace of compunction. If we go low enough down the social scale we find the freest utterance of feelings, and it is only when, in retracing our steps, we arrive at a certain stage of culture that we discover signs of an active emotional restraint. Where this self-control is defective we have Mr. Spencer's secondary emotional signs. Higher up, among a few specially cultivated persons, the acquisition of this power of concealment appears to be complete, and we have a type of mind capable of a prolonged external serenity unruffled by a gust of passionate impulse. The survey of these facts at once prompts the question whether the expression of our feelings by smile, vocal changes, and so on, is destined to disappear with a further advance of social organization. To attempt to answer such a question directly and briefly would perhaps betray too much confidence. We may, however, seek to define the various paths of inquiry to be pursued before a final answer can be arrived at, and to hint at the probabilities of the problem under its various aspects.

First of all, then, with respect to the distinctly unsocial feelings, the answer seems to be tolerably clear. It being generally allowed by biologists that the looks and gestures accompanying anger, jealousy, and pride, are simply survivals of hostile actions, the nascent renewal of an attitude preliminary to attack, it is natural that they should appear only in transitions of society from a barbaric to a civilized condition. When the age of destructive conflict, individual and racial, shall have become the curious research of antiquaries, it may be presumed that any bodily movements known to have grown out of these struggles will cease from sheer desuetude. Indeed, one may perhaps, without too optimist a bias, refer to the fact that all the stronger manifestations of anger and malice have already become unfamiliar in real life, so that when we see their imitations on the stage they are apt to appear ridiculously forced. The better part of modern society has put such a ban on the ugly signs of rage that our only means of discovering traces of this passion in a man is some incompletely suppressed emotional movement, or some too violent effort to command the muscles of expression. After many more generations shall have practised the difficult art of noiselessly crushing out with the foot an incipient wrath, it will be hard if such offenses to the eye as frowning brow and scornful mouth do not entirely disappear.

But the progress of social refinement probably affects other expressions than those of the distinctly hostile sentiments. It tends to