This is one of my favorite 'old style', or vintage looking pieces of magic. Some versions come with two card images on the glass, some with only one. A magician I know suggested using a monocle from an antique shop, and using 'Ghost Glass Impromptu' to get a card on the glass same, easily done with a pair of antique spectacles. Both, lend themselves to a potential patter, about eye wear that belonged to an old magician. The disc that comes with this version, can be woven into some kind of story, but not as easily I think than with eyeglasses, or, a wine glass.
The fleeting image invoked by the spectator's breath, is very ghostly to the eye. Once gone, it's invisible. It helps to do this effect in good lighting, and close to the spectator, as the image can fade quicker than they may see it.
One handling that I use, goes all the way back to a Joseph Dunninger trick, included in Scarne on Cards. Dunninger's version, a huge mystery in his time, used a crystal ball instead of a glass disc. His force is easy, and effective-though there are many forces one may use.
This is an usual effect-and, NOT something that people may have ever seen before, thus, the memorability factor is high. I also love it, because it seems like a trick that is connected to the old days of 'mediums', slate writing, crystal balls, and vaudeville-back when magicians would wear monacles,.. along with spats and tuxedos. Beautiful, haunting trick,

For $15 you have a little miracle!
The spectators can look and look at the glass but they will find nothing!
If you have $15 spare then buy this, even if it's only to amuse yourself.