Note from Dr Leslie Armstrong to Sherlock Holmes

SIR [it ran]:
I can assure you that you are wasting your time in dogging
my movements. I have, as you discovered last night, a
window at the back of my brougham, and if you desire a
twenty-mile ride which will lead you to the spot from which
you started, you have only to follow me. Meanwhile, I can
inform you that no spying upon me can in any way help Mr.
Godfrey Staunton, and I am convinced that the best service
you can do to that gentleman is to return at once to London
and to report to your employer that you are unable to trace
him. Your time in Cambridge will certainly be wasted.
Yours faithfully,
LESLIE ARMSTRONG