I know this is going to sound strange but trust me it works and will not harm the optics like chemicals. I learned this from a fellow tec when i worked for the Kenwood service center some years ago and have used this method on CD players and optical mice over and over for years and it has yet to fail. Keep in mind that what you most likley have is fuzzy or fogged optics. The solution is "spit" and no i'm not kidding, assuming you're not into chewing tobacco or have a mouth full of tootsie rolls this method works fine. Find some of those little medical swabs at any pharmacy ( in a pinch a q tip will do but be carefull of the fuzz) turn the mouse over grab a swab and spit on the end and use it to clean the lens, then take a clean swab and polish the lenses gently. The reason you shouldn't use alcohol is that it attacks some plastics (ruining them) and (assuming it didn't melt it) it also attracts water vapor as it dries leaving a film on the lens.
Good old spit, cheap, works well and is generally considered non toxic.... unless of course it's swallowed in small ammounts over a Looooong period of time.
Hope this helps.