Who is the foil in "The Adventure of the Speckled Band?"

The definition of a foil is "a secondary character who
contrasts with a major character." A foil often has a great deal in common with the
character he is contrasted with, but it is the dissimilarities of the two characters
that make the qualities and characteristics of the main character stand out so visibly,
giving important information to the reader about the main character. (Examples given are
Hamlet and Fortinbras, as well as Hamlet and Horatio, and Hamlet and Laertes. Two of
these men are good men; only the third is corrupt.)

I would
expect that Dr. Watson is something of a foil (though perhaps not
the main foil) in the Sherlock Holmes' mysteries. Watson, like the reader, is of average
intelligence who sees what is before him as does Holmes. However, it is by comparing the
reactions and interpretations of both men that the reader is led to witness Holmes'
obviously superior abilities at observation and deductive
reasoning.

For instance, Watson sees the saucer of milk,
the "leash," the ventilator, the fake bell pull, and the bed secured to the floor, under
the ventilator just as Holmes does. The reader shares Watson's instinct that each of
these is an important clue, but no sense can be made of the significance of the clues
without Holme's brilliance, and knowledge of the criminal
mind.

However, in terms of the mystery, the criminal in the
story is often the most effective foil as he is usually as brilliant as Holmes' but his
evil nature generally pushes him to make a mistake that allows Holmes' to overcome and
best the villain.

Whereas Watson is a commonplace figure
each Holmes mystery, much less is known about our villain, perhaps because we cannot be
sure it is Dr. Roylott until the very end. Foreshadowing is evident as we see Roylott's
temper when he barges into Holmes' residence, threatening verbally and physically (with
the poker)—drawing a comparison between the two men in terms of their strength
personalities and physical prowess. Both are equally matched against the other alhtough
the reader is unaware of just how dazzling Roylott's intellect is until the
end.

However, Roylott is no match for Holmes. Roylott
allows his desire for Helen's money to cloud his thinking, and he places himself at risk
by selecting the very tools he chooses to bring about Helen's (and her sister's)
demise.

Choosing between the two, in light of other
Sherlock Holmes mysteries I have read, it stands to reason that the
primary foil for Holmes will always be
his arch-villain in each story.