When you looked through them they had the properties of plain glass; when you looked at them they had the appearance of lenses.

But only one kind of glass possesses these properties; namely, that which, like an ordinary watch-glass, has curved, parallel surfaces.

But for what purpose could a person wear 'watch-glass' spectacles?
Clearly, not to assist his vision.

The only alternative is disguise.
"The properties of these spectacles introduce a very curious and interesting feature into the case.

To the majority of persons, the wearing of spectacles for the purpose of disguise or personation, seems a perfectly simple and easy proceeding.