I just read Zhisou's excellent latest post, a series of interconnected musings which I would recommend you have a look at (hope you don't mind me linking to it, Z).
One of the themes he touches upon is that of staring at the sun despite/because/regardless of the dangers involved: this triggered a memory for me, albeit only tangentially related to the aforementioned (aforelinked?) post.
Years ago I did numerous photography courses and projects, during my art college days. I loved photography, both the creative and technical aspects of it: generally speaking I was as happy spending hours in the darkroom as I was being out and about armed with camera and rolls of film (this was, of course, in the days before digital photography).
Often there'd be two or three of us crammed into a relatively small darkroom space - either because we were collaborating on a project, because no other darkrooms were available, or because we were just dossing about for the sake of it. On one such occasion, my self and two fellow students had just wrapped up an hour or two of developing photographs, and were about to switch the main light on (since it was now safe to do so without ruining any films or light sensitive paper).
Somehow, whoever was making for the light switch managed to accidentally trigger off the camera flash unit that he'd also just picked up. At first we cursed him due to the sudden and startling (not to mention blinding) flash, the effect was very disorientating - all bright colours and amorphous shapes suddenly burned onto the retina. It was almost like being punched, and the three of us collectively spent a moment trying to compose ourselves again.
Then a most weird sensation occurred. As I was looking round the darkroom (still pitch black - we hadn't found the light switch yet), I noticed, clear as day, a pair of hands floating around the room. Wherever I looked, up or down, there they were.
I quickly realised that my hands must have been in my line of sight when the flash went off. Now, after its momentary, almost explosive visual effect had subsided, what remained seared into my retina was what the light had hit when the flash went off - the image of my hands rendered with startling, monochrome clarity.
The other two students had noticed the same thing happening, corresponding to what had been in their own respective lines of sight at the moment in question.
So we didn't switch the light on. We waited a few moments for the images to fade, and then triggered the flashgun again. The same bright burst for a fraction of a second, quickly subsiding to reveal a monochrome imprint of whatever we were looking at.
Regardless of the potential for damage to our eyesight, we carried on playing with this. For example, if you looked down at your leg when the flash went off, and then looked upwards, you'd see your leg floating right above you - or at eye level if you were looking straight ahead. The flash rendered the images like ice sculptures - beautiful and clear and black and white, which added to the eeriness of the sensation.
Sometimes it would be genuinely unsettling - triggering the flash when someone's face was about two feet away from yours was to see a horror-mask flying around the room wherever you looked. Nonetheless, there was something quite addictive about the whole thing. It was like we were creating momentary, frozen scenes - stark, other-wordly, and lasting for just a few seconds before fading away forever.
Finally - after something like half an hour - we decided to stop and head out of the darkroom, blinking. Did our eyes ache? I can't remember. We repeated the whole thing numerous times though whenever we were back in the darkroom: despite wondering about the potential for retinal damage (it was certainly headache-inducing) there was too much novelty value not to give it another go.