I don't know whether these are useful, useless, silly, or previously invented by others. But they're here.
There have been systems to create images on eyeglasses, with special lenses to allow the eye to focus very closely, on the actual surface of the eyeglass lens. Other systems project an in-focus image onto the retina of the eye.
This idea came to me in a dream on 24 Sep 2004. I dreamed that a guy was working on some type of machine, and he needed to enter a password or numeric code. For whatever reason, the machine didn't have a communications system to allow him to contact someone for the information.
He called with a cell phone, and, rather than get the information on the cell (e.g. via text message), it appeared on his eyeglasses. He then looked at the reflective metal surface of the machine to read the message on his eyeglasses.
So, the invention is an eyeglass lens with a thin-film imaging layer (e.g. some type of LCD). Normally, the imaging layer is transparent, so it's invisible.
On demand, one can create an image (I had assumed a few words of text) on the eyeglass surface, in reverse (the message would be readable from within the glasses if you could focus on it). The image itself would not be opaque, so one could see through it as if the lens were dirty. If you look into a mirror, you'd be able to see the image on your eyeglasses.
Consider writing on eyeglasses with a crayon or grease pencil. You can't read the words when wearing the glasses, but you can look through the writing and read the words by looking into a mirror.
No special mechanism is needed to focus on the close-up image, because the image appears twice as far away as you are from the mirror.