In "Dusk," by Saki, explain the role played by the dusk and the cake of soap?

Significantly, dusk is described as the hour of the
defeated by Norman Gortsby.  It is the hour of "gloaming" that brings out those who
suffer fallen fortunes or lost hopes.  They come at dusk to avoid the scrutiny of those
who wonder what has occurred, for it is the time that they will not be noticed.  Many of
the people who come out at this time have lost money, loves, or position, so they appear
when they will be little noticed.

Thus, as Gortsby sits
amid this milieu of discouragement and defeat, there is an older gentleman on the other
end of the bench.  After a time, he gets up and leaves so that the bench is vacated. 
Shortly, however, the young man who tells the tale of misfortune joins Gortsby on the
bench.  The cynical Gortsby doubts the young man's story about leaving his hotel for a
bar of soap and not being able to find his way back, because the speaker does not have
the visible proof of the veracity of his story; namely, the soap.  Even when the young
man feels his pockets for the soap, Gortsby doubts him.  So, the young man "flits" away
down the path.

As he, too, rises to depart, Gortsby sees on
the ground a bar of soap, which has evidently fallen from the pocket of the young man. 
Gortsby picks it up and hurries after the young man.  "The important witness to the
genuineness of your story has turned up," Gortsby tells the young man, who hastily
pockets the soap.  In contrition, Gortsby offers the young man some money with his card
so that the money can be returned.  With a "catch in his voice," the young man says,
"Lucky thing your finding it."

Returning to his bench,
Gortsby congratulates himself on catching up to the young man, but he chides himself,
"It's a lesson to me not to be too clever in judging by circumstances."  Of course, the
irony to the ending is that the old man returns to search for  his cake of soap; Gortsby
learns that he has been tricked after all and his first judgment was the correct one. 
Indeed, Gortsby does belong on the bench in the twilight, "the hour of the
defeated."