fying
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, PP