154 Chapter 10
made substantial changes to his argument by foregrounding the well-known controversy
between medium and emission theories as the ground on which his analogy, and his
ensuing new theory, stood. Still, his original statement clarifies the preeminent significance
of this analogy in his thinking.^7 As he clarified his argument, Euler also sharpened the
contrast between medium and emission theories, which until then had often (as we have
seen) been combined in various ways, rather than being considered entirely exclusive of
each other. Euler used the analogy with sound to ground his polemic for his medium theory.
To do so, Euler addressed head on a difficulty that probably led his 1744 audience to
doubt his argument, the very difficulty Newton had emphasized: whereas sound pulses
entering a room through an opening penetrate the whole space, light rays do not seem to
behave similarly. Euler ’ s reply reasserts the power of the analogy with sound, which he
contends that Newton misunderstood. Euler argues that Newton ought to have compared
the optically opaque room with an acoustically “ opaque ” barrier because sound can pass
even through normal walls, not just through an opening between rooms. Only a perfectly
soundproof barrier could rightly be compared with a visually opaque wall, though Euler
acknowledges the practical difficulty in constructing such an ideal acoustic barrier. His
insistence on the sound/light analogy brings forward the precise limitation of Euler ’ s views
that Young will address in the next chapter. Euler asserted that sound had not been observed
to spread out laterally in a room but propagates linearly, as does light. In his view, one
sound pulse could not prevent or interfere with another so as to allow sidewise spreading.
Indeed, the question of the spreading of sound propagated through a medium remained
controversial for the next half century; Euler was relying on the absence of what he con-
sidered sufficient evidence for spreading, but he also seemed to have used the analogy in
reverse, applying the straight-line propagation of light rays to sound.
This example shows the multiple possibilities that lay within the application of the
sound – light analogy. Rather than imposing the later perspective that Euler simply erred,
my point is more that the example of sound was so strong for him (as for Newton himself)
that it remained potent even when its conclusions may seem paradoxical. Yet there was no
paradox for Euler: his argument about sound-transmitting walls seemed to answer New-
ton ’ s assertion neatly, nor did Euler have any positive information on sound propagation
that would have differed from what was common knowledge about the rectilinear propaga-
tion of both sound and light. Euler fortified his argument with several others against
Newtonian emission (such as that it would deplete the sun ’ s matter and would preclude
the possibility of transparent materials). Consistently, he found no evidence that a beam
of sound or of light could interfere in any way with another such beam, which an emission
theory would portray as the collision between the emitted particles. Ironically, this impor-
tant advocate of wave theory seemed to overlook what, in the sequel, seemed to others its
most salient feature, interference. He did so as much because of his arguments for the
lack of interference between sound sources as because of his arguments for the lack of
interference between light beams: above all, he maintained the analogy between them.