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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.

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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.