can be followed for a considerable time.
Birds' eggs can be incubated in a warm chamber, and by removing a
portion of the shell and replacing it by an unbroken piece from
another egg, it becomes possible to follow the daily development of
the chick and to experiment upon it. As early as the ninetieth hour of
incubation, spontaneous "impulsive" movements may be observed, taking
place apparently without any external stimulus as a cause, and at a
time when no muscles or nerves have as yet been developed. After the
occurrence of these spontaneous movements, and at the earliest on the
fifth day of incubation, movements are observed to result from the
application of mechanical, chemical, and electrical stimuli. In order
to observe these the eggs must be allowed to cool down until all
spontaneous movements have ceased. From the tenth to the thirteenth
day more complicated and reflex actions occur on the application of
stimuli, as, for instance, movements of the eyelids, beak, and limbs;
and if the stimuli are strong, reflex respiratory movements. These
reflexes make their appearance before any ganglia have become
differentiated. Prof. Preyer considered himself justified in
concluding from this that ganglia are not essential for the liberation
of reflex actions. He intends, on some future occasion, to give a more
detailed account of these experiments, and of the conclusions which
may be drawn from them. In the discussion which ensued the conclusions
of the speaker were contested from many sides.

* * * * *

IRIDESCENT CRYSTALS.[1]

[Footnote 1: Abstract of the Friday evening lecture delivered by
Lord Rayleigh, F.R.S., at the Royal Institution, on April 12,
1889.]

By LORD RAYLEIGH.

The principal subject of the lecture is the peculiar colored
reflection observed in certain specimens of chlorate of potash.
Reflection implies a high degree of discontinuity. In some cases, as
in decomposed glass, and probably in opals, the discontinuity is due
to the interposition of layers of air; but, as was proved by Stokes,
in the case of chlorate crystals the discontinuity is that known as
twinning. The seat of the color is a very thin layer in the interior
of the crystal and parallel to its faces.

The following laws were discovered by Stokes:

(1) If one of the crystalline plates be turned round in its own
plane, without alteration of the angle of incidence, the
peculiar