The Grit-advocate., 1903-10-23

l uhl lN''lI position HIS ht'lt utter
mined by th" direction Of the light
waves and not merely by differences
of illumination has been established
by several methods of proof.
\ DEMONSTRATIONS.
In the water i'll n shallow dish are
placed 4 number of specimens which
manif"st the tendency 10 swim away
from the light. over the dish is placed
Il hollow prism filed with II somewhat
Opaque liquid. In the shutter is Il
window through which the apparatus
is Illuminated and light from all other
sources is excluded. With this ar
rangerNent the darker side of til" dish
is the one toward the window. be
cause the thick side Of the prism cuts
out Il larger proportion of the light:
hut the direction of the rays entering
tie water IS practically the same as
it would be with the prism removed.
Without the intcrccptiug prism. the
animals, being negatively hcliotropic.
collect ut the dark side of the dish
thai is. the side farthcst from the win
dow When the prism is put into place
the animals do not collect at the dSrk
uhc window side of the dish. but
again proceed directly in the course
of tho light rays-that is TO say, they
swim from the window side, which,
by his arrangement. is relatively dark,
10 the room side. which is relatively
bright. BY similar methods ii has been
..l. sb,nf f,;b ,,th nj a ~
positively heEotropic: they NOV" to-
ward the source IF light. not because
tie direction of the light rays deter
mines their movements; and this "c-
ours even when the movement takes
them from bright to dim ii::i.t.
BEYOND ANIMAL CONTROI..
II is no longer rational to speak of
such movements rs voluntary. or to
classify the forms which exhibit them
by such anthropomorphic terms AS
DARK loving III iight-loviuS. The pro
vcTDia! moth does not crcma1c itself
in the candle because Of its excessive
folly. but bccaus" Of its intense hello
trUpl~ ~m. If the moth is Il conscious and
moral agent. its sin does not consist
in The act Of rushing into the flame.
bur n goin" near enough 10 allow the
ligt rays to set in operation processes
beyond its control. Here the social re
former may easily append moral.
ON" of the most curious facts in the
effect of light is that upon certain ami
mails its action is reversible. Thus the
larvae of some of the barnacleS are
positive to dim and negative to bright
light Under the influence of the rays
which penetrate the water they are
compelled to swim toward the surface.
Before reaching it. however. the in
creased intensity Of the light changes
them TO the negative condition and
they swim downwardS. until in the
weak illumination they again become
positive. Thus they 20 dancing up
and down, the plaything of physical
forces.
POSITIVE AND NEGATIVE.
TIL" larvae Of n certain marine worm
are still more suggestive examples of
this kind of change. AT ordinary room
tcluDerature they collect on the side
of the aquarium farthcst from the win
dow If the temperature is lowered
to i degrees c.. or e degrees c.. they
migrate to the window side. Allow
the water to become warmer than
: do@rees c.. they 20 back TO tie dark
side There are other animals which
are ucgative in the presence of a sutti-
cieut oxygen supply. but bccom" posi-
tive N its absence. and there are forms
whose heliotropism may be reversed
by change in the concentration of the
sea water.
It evident that through one agent,
ligh', nature can exert an ine1orable