"But the judgment complained of, namely,
sentence to a term of imprisonment, depends upon
the exercise of a judicial discretion by the
court imposing it. The manner in which an
appeal against an exercise of discretion should
be determined is governed by established
principles. It is not enough that the judges
composing the appellate court consider that, if
they had been in the position of the primary
judge, they would have taken a different course.
It must appear that some error has been made in
exercising the discretion. If the judge acts
upon a wrong principle, if he allows extraneous
or irrelevant matters to guide or affect him, if
he mistakes the facts, if he does not take into
account some material consideration, then his
determination should be reviewed and the
appellate court may exercise its own discretion
in substitution for his if it has the materials
for doing so. It may not appear how the primary
judge has reached the result embodied in his
order, but, if upon the facts it is unreasonable
or plainly unjust, the appellate court may infer
that in some way there has been a failure
properly to exercise the discretion which the
law reposes in the court of first instance. In
such a case, although the nature of the error
may not be discoverable, the exercise of the
discretion is reviewed on the ground that a
substantial wrong has in fact occurred."