INFLUENCE OF REPOSE ON THE RETINA.
Some interesting researches have lately been published in an Italian journal concerning the influence of repose on the sensitiveness of the retina (a nervous network of the eye) to light and color. The researches in question—those of Bassevi—appear to corroborate investigations which were made some years ago by other observers. In the course of the investigations the subject experimented upon was made to remain in a dark room for a period varying in extent from fifteen to twenty minutes. The room was darkened, it is noted, by means of heavy curtains, through which the light could not penetrate. After the eyes of the subject had thus been rested in the darkness, it was noted that the sensitiveness of his sight had been increased threefold. The mere sense of light itself had increased eighteen times. It was further noted that the sensitiveness to light rays, after the eye had been rested, was developed in a special order; the first color which was recognized being red, then followed yellow, while green and blue respectively succeeded. If color fatigue was produced in the eye by a glass of any special hue, it was found that the color in question came last in the series in point of recognition. The first of these experiments, regarded from a practical point of view, would appear to consist in an appreciation of the revivifying power of darkness as regards the sight. The color purple of the retina is known to become redeveloped in darkness; and it is probable, therefore, that the alternation of day and night is a physical and external condition with which the sight of animals is perfectly in accord.
SUN DIALS.
An article on the subject, recently published by us, has gained for us the communication of two very interesting sun dials, which we shall describe. The first, which we owe to the kindness of General Jancigny, is of the type of the circular instrument, of which we explained the method of using in our preceding article. The hour here is likewise deduced from the height of the sun converted into a horary angle by the instrument itself; but the method by which such conversion operates is a little different. Fig. 1 shows the instrument open for observation. We find here the meridian circle, M, and the equator E, of the diagram shown in Fig. 3 (No. 4); but the circle with alidade is here replaced by a small aperture movable in a slide that is placed in a position parallel with the axis of the world. Upon this slide are marked, on one side, the initials of the names of the months and on the other side the corresponding signs of the zodiac. The sun apparently describing a circle around the axis, PP¹, the rays passing through a point of the axis (small aperture of the slide) will travel over a circular cone around such axis. If, then, the apparatus be so suspended that the circle, M, shall be in the meridian, the slide parallel with the earth's axis, and the circle, E, at right angles with the slide, the pencil of solar light passing through the aperture will describe, in one day, a cone having the slide for an axis; that is to say, concentric with the equator circle. If, moreover, the aperture is properly placed, the luminous pencil will pass through the equator circle itself; to this effect, the aperture should be in a position such that the angle, a (Fig. 3, No. 4), may be equal to the declination of the sun on the day of observation. It is precisely to this end that the names of the months are inscribed upon the slide....
The accessories of the instrument are as follows: A ring with a pivot for suspending the meridian circle, and the position of which, given by a division in degrees marked upon this circle, must correspond with the latitude of the place; two stops serving to fix the position of the equator circle; finally the latitude of various cities. The instrument was constructed at Paris, by Butterfield, probably in the last quarter of the eighteenth century.
The second instrument, which is of the same nature as the cubical sun dial—that is to say, with horary angle—is, unlike the latter, a true trinket, as interesting as a work of art as it is as an astronomical instrument. It is a little mandolin of gilded brass, and is shown of actual size in Fig. 2. The cover, which is held by a hook, may be placed in a vertical position, in which it is held by a second hook. It bears in the interior the date 1612. This is the only explicit historic datum that this little masterpiece reveals to us. Its maker, who was certainly an artist, and, as we shall see, also a man of science, had the modesty not to inscribe his name in it.
No. 2 of Fig. 3 represents the instrument open. It rests upon the tail piece and neck of the mandolin. The cover is exactly vertical. The bottom of the mandolin is closed by a horizontal silver plate, beneath which is soldered the box of a compass designed to put the instrument in the meridian, and carrying upon its face an arrow and the indications S. OR. M. OC., that is to say, "Septentrion" (north), "Orient" (east), "Midi" (south), "Occident" (west). One of the ends of the needle of the compass is straight, while the other is forked. It is placed in a position in which it completes the arrow, thus permitting of making a very accurate observation (Fig. 2, No. 3). Around the compass, the silver plate carries the lines of hours. It is perfectly adjusted, and held in place by a screw that traverses the bottom of the instrument. In front of the compass it contains a small aperture designed to permit of the passage of the indicating thread, which, at the other end, is fastened to the cover. The silver plate is not soldered, in order that the thread may be replaced when it chances to break. On the inner part of the cover are marked in the first place the horary lines, traversed by curves that are symmetrical with respect to the vertical and having the aspect of arcs of hyperbolas. At the extremity of these lines are marked the signs of the zodiac. At the top, a pretty banderole, which appears at first sight to form a part of the ensemble of the curves, completes the design. Such is this wonderful little instrument, in which everything is arranged in harmonious lines that delight the eye and easily detract one's attention from a scientific examination of it. Let us enter upon this drier part of our subject; we shall still have room to wonder, and let us take up first the higher question.