It's methyl-alcohol (wood alcohol) that converts to formic acid in the retina and makes you blind. Happened a lot during prohibition and you hear of it now and again in Eastern Europe when people add methyl-alcohol in the form of anti-freeze to spirits like vodka.
When you go to some places on holiday you can buy bottles of whisky, brandy etc for less than a quid it is often alcohol with an added flavouring to try and make it taste like a proper drink. I think it's usually ethyl-alcohol but I would never buy any in case it's the other kind!