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another. There are many cases o' this but none
mora striking thsn the one Just noticed


A wish has been expressed that ,hould explain
mere fully circumstance 10 which alluded oriet-
Is in former lecture namely, The effect produced
on the retina of the eve By light. explained the
principle Of light as estas ished b. modern p1ysics
and tie nsression Of the different col1rs upon the
retina The physical principle on which hght de-
psnds was far long while, and Is still in dispute
among philosophers. One sect maintains that .ighr
i. physical emanatioR from luminous body.
which passes through space at the rate Of 200 600
miles In a second, reaches the eve affects the reil-
na and produces IN impression IN the mind. They
hold tina; the white light OF the Sun i compose OF
particle, ct different kinds each producing the no.
tion OF different color. This theory has been IN
dispute and 1. found 10 De insufficient Tor the eN-
panetion OF certain phenomena discovered oy moi
ern science. Tas theory, however was maintain
ed by NEWTON


Another heory. which i generally received
i this: X supposes that the whole universe i MI
ed with fsid called ather. extremely subtle and
elastic and that the lminou. body produces the
eiisc y imparing TO thi, ather certain pulsa-
iiou, precisely similar TO that ef sound, TO which
have already referred. These vibration, are sup
posed to be- transmitted 10 the eve with The veloci
y already mentioned. Having entered the eye. II


uses the retiua 70 vibrate just q doe, the ear
drum n hearing, only these pusation- are Infinite
Is more rapid and delicate than those OF soAnd.-
IAi- theory mainian- that the colors Of the spec
trum. a, red orange, blue, Ac are nothing more
than the effects OF greater le- rapidiiy OF vbrs
non Ii the retlna pusai"s At rate. red viii
be seen ii al another blue. &c. tn, - the gen
era outline of the theory now generally received IN
preference 10 the CorppScnf!ar theory. which was
neli by NEwToS. bat which J.. insufficient to eX-
plain many of the phenomena observed which the
undu.ating theory perfectly accounts for. What
all these phcaamena arr cannot explain in full
! will, however, mention one Of them, which Is
among the most remarkable.


II TWO beam, OF light be admitted through small
aperiures in screen, and 5e made to each
other under certain circumstances. that they tall
upon the same point, you would naturally expect
that that point would be twice light As If but one
beam fell upon it. According TO The Corpu,cular
theory which holds light be material substance,
tie more OF il there was accumulated upon any
point the greater would be is illumination. But
II has been shown by modern science that, instead
OF this result, the t~o beams destroy each other,
and black spot /, seen Al The point OF intersection.
If either of the beams be Intercepted the spot be-
comes luminous but fboth be allowed to fall upon
it together it becomes black: either of the TWO
will illuminate it-b1,h together produce darkness.
Now the Corpu,cular theory fails account tor this
,nenomenon: but IL is clearly explained by the the
ory Of an unduatng medium Only give Inc
explanation in general way, az thus: 1t Is neces
rary to show that it i. possible for two system, of
TO obiierate each other. in order TO The eN
PLANTATION; for il the presence OF waves i, essential
20 illumination, any thing which destroy, them must
produce darkness. Now if we suppose two sys