But, to a person of normal eyesight, it is nothing of the kind.

For, if he wears spectacles suited for long sight he cannot see distinctly through them at all; while, if he wears concave, or near sight, glasses, the effort to see through them produces such strain and fatigue that his eyes become disabled altogether.

On the stage the difficulty is met by using spectacles of plain window-glass, but in real life this would hardly do; the 'property' spectacles would be detected at once and give rise to suspicion.
"The personator is therefore in this dilemma: if he wears actual spectacles, he cannot see through them; if he wears sham spectacles of plain glass, his disguise will probably be detected.

There is only one way out of the difficulty, and that not a very satisfactory one; but Mr.
Weiss seems to have adopted it in lieu of a better.

It is that of using watch-glass spectacles such as I have described.
"Now, what do we learn from these very peculiar glasses?
In the first place they confirm our opinion that Weiss was wearing a disguise.