"To return to Weiss. He was clearly heavily disguised, as we see by his unwillingness to show himself even by the light of a candle. But there is an item of positive evidence on this point which is important from having other bearings. It is furnished by the spectacles worn by Weiss, of which you have heard Jervis's description. These spectacles had very peculiar optical properties. 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. 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.
