March 22, 1873.] 
THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
other—they also add themselves together, and we have 
an augmented luminous effect. The same occurs when 
one system of waves is any number of whole wave-lengths 
in advance of the other. But if the one system be half a 
wave-length, or any odd number of half wave-lengths in 
advance, then the crests of one system fall upon the 
sinuses of the other system; the one system, in fact, tends 
to lift the particles of ether at the precise places where 
the other tends to depress them ; hence, through their 
joint action, the ether remains perfectly still. This still¬ 
ness of the ether is what we call darkness, which corre¬ 
sponds, as already stated, with a dead level in the case of 
water. 
It has been stated, with reference to the colours of 
absorption, that the formation of natural bodies is selec¬ 
tive, not creative ; that what they did was to pick out 
certain constituents of the white solar light for extinction, 
this causing them to appear in the colours of the unextin¬ 
guished light. It must at once flash upon your minds 
that, insomuch as we have in interference an agency by 
which light may be self-extinguished, we have in it the 
conditions for the production of colour. But now the 
question faces us, How is it that certain constituents are 
quenched by interference, while others are permitted to 
remain ? This is entirely due to the difference in the 
lengths of the waves of light. 
The subject is most easily illustrated by the class of 
phenomena which first suggested the undulatory theory 
to the mind of Hooke. These are the colours of thin 
films of all kinds, which are known to men of science as 
the colours of thin plates. In this relation no object in 
the world possesses a deeper scientific interest than a com¬ 
mon soap-bubble. And here, let me say, emerges one of 
the difficulties which the student cf pure science encoun¬ 
ters in the presence of “practical” communities like those 
of America and England; it is not to be expected that 
such communities can entertain any profound sympathy 
with labours which seem far removed from the domain of 
practice. Imagine Dr. Draper spending his days in blow¬ 
ing soap-bubbles and in studying their colours ! Would 
you show him the necessary patience, or grant him the 
necessary support ? And yet be it remembered it was 
thus that Newton spent a large portion of his time ; and 
that it was on such experiments that has been founded a 
theory, the issues of which are of incalculable importance 
to the human race. 
Whence, then, are derived the colours of the soap- 
bubble ? Imagine a beam of white light impinging on 
the bubble. When it reaches the first surface of the film 
a known fraction of the light is reflected back. But a 
large portion of the beam enters the film, reaches its 
second surface, and is again in part reflected. The waves 
from the second surface thus turn back and hotly pursue 
the waves from the first surface. And, if the thickness 
of the film be such as to cause the necessary retardation, 
the two systems of waves interfere with each other, pro¬ 
ducing augmented or diminished light, quadrupling it, or 
totally extinguishing it, as the case may be. But, inas¬ 
much as the waves of light are of different lengths, it is 
plain that, to produce self-extinction in the case of the 
longer waves, a greater thickness of film is necessary than 
in the case of the shorter ones. When, therefore, the red 
is quenched, the blue and green are not quenched : hence 
the production of colour in the case of thin plates. 
The experimental illustrations to be introduced in the 
lecture are the colours of oil upon water ; the colours of a 
film of air inclosed between two glass plates ; the colours 
of films of oxide used as a guide in the tempering of 
polished steel; the colours of the soap-bubble by trans¬ 
mission and reflection; finally, the celebrated experi¬ 
ment by which Newton determined the thickness of a 
soap-bubble from its colour, and known as the experiment 
of Newton’s Rings. To account for these rings was the 
greatest difficulty that Newton ever encountered. He 
quite appreciated the difficulty ; over his eagle-eye there 
was no film, no vagueness in his conceptions. At the 
very outset his theory was confronted by the question. 
Why, when a beam of light is incident on a transparent 
body, are some of the light-particles reflected and some 
transmitted ? Is it that there are two kinds of par¬ 
ticles, the one specially fitted for transmission and the 
other for reflection ? This cannot be the reason ; fox, if 
we allow a beam of light which has been reflected 
from one piece of glass to fall npon another, it, as a 
general rule, is also divided into a reflected and a trans¬ 
mitted portion. Thus the particles once reflected are 
not always reflected, nor are the particles once trans¬ 
mitted always transmitted. Newton saw all this ; he 
knew he had to explain why it is that the self-same 
particle is at one moment reflected and at the next 
moment transmitted. It could only be through some 
change in the condition of the particle itself. The self-¬ 
same particle, he affirmed, was affected by “ fits ’ ol 
easy transmission and reflection. 
If you are willing to follow me while I unravel this 
theory of fits, the most subtle, perhaps, that ever entered 
the human mind, I think the intellectual discipline 
will repay you for the necessary effort of attention. 
Newton was chary of stating what he considered to be 
the cause of the fits, but there cannot be a doubt that 
his mind rested on a mechanical cause. Nor can ihere- 
be a doubt that, as in all attempts at theorizing, he was- 
compelled to fall back upon experience for the materials 
of his theory. His course of observation and of thought 
may have been this : From a magnet he obtained the 
notion of attracted and repelled poles. What more 
natural than that he should endow his light-particles 
with such poles ? Turning their attracted poles toward 
a transparent substance, the particles would be sucked in 
and transmitted ; turning their repelled poles, they would 
be driven away or reflected. Thus, by the ascription of 
poles, the transmission and reflection of the self-same 
particle at different times might be accounted for. 
At the point which we have now attained, Newt on A 
ingenuity in preserving a theoretic conception entire 
reached its climax. Regard these rings of Newton as 
seen in pure red light ; they are alternately bright and 
dark. The film of air corresponding to the outermost 
of these rings is not thicker than an ordinary soap- 
bubble, and it becomes thinner on approaching the 
centre; still Newton, as I have said, attempted to- 
measure the thickness of every ring, and to show the 
difference of thickness between ring and ring. How 
he did this may be thus made plain to you. Suppose 
the water of the ocean to be absolutely smooth; it 
would then accurately represent the earth’s curved 
surface. Let a perfectly horizontal plane ^ touch the 
surface at any point. Knowing the earth’s diameter, 
any engineer or mathematician could tell how far the 
sea’s surface will lie below this plane, at a yard, at 
ten yards, at 100 yards, at 1,000 yards’ distance from 
the point of contact of the plane and the sea. It is 
common, indeed, in levelling operations, to allow for 
the curvature of the earth. Newton’s calculation was 
precisely similar. His plane glass was a tangent to his 
curved one. From its refractive index and focal distance 
he determined the diameter of the sphere of which his 
curved glass formed the segment, he measured the 
distances of his rings from the place of contact, and 
he calculated the distance between the tangent plane and 
the curved surface, exactly as the engineer wouid cal¬ 
culate the distance between his tangent plane and the 
surface of the sea. The wonder is, that where such 
infinitesimal distances are involved, Newton, with the 
means at his disposal, could have worked with such 
marvellous exactitude. 
Now mark the result. For thejsake of convenience let 
us call the thickness of the film of air corresponding to 
the first dark ring d ; then Newton found the distance 
corresponding to the second dark ring 2 d ; the thickness; 
corresponding to the third dark ring 3 d ; the thickness 
corresponding to the tenth dark ring 10 c, and so on 
