Friend of mine in the UK saw a program about this, and says that the researchers
repeated successfully the procedure they claim Leonardo could have followed,
using egg, some salt and urine...
The weakest part of the argument is that they claim Leonardo is also THE SUBJECT
of the "photo", and they cannot explain why it should be him and not somebody
else who discovered this.
I am amongst the sceptics, though if it can be proved that an Italian produced a
permanent photograph three centuries before the French, well.... :-)
Oliv