by which methanol causes blindness
is as follows :-
There is a enzyme (an alcohol dehydrogenase) in the eye which is responsible for turning retinol ; an alcohol, into retinal ; an aldehyde. This enzyme also works on other alcohols, and with methanol gives methanal, a very toxic compound. As little as 10 grams of methanol can cause blindness....
There is however a treatment if you catch the poisoning fast enough. Because the dehydrogenating enzyme also works with ethanol, you can prevent a build up of methanal by giving a large dose of ethanol to the patient. This uses up the enzyme in the eyes, reducing the amount of methanal it can produce. As the methanol is water soluble it's eventually excreted by the body, helped no doubt by the diuretic effect of ethanol.
You're then left with probably one of the worst hangovers ever experienced by anyone, ever!
Source :- The Right Chemistry, Jeffery Hancock, Hodder & Stoughton ISBN 0-340-70194-3