Oxford Road station.
If you can't get a train to there, the next nearest is Picadilly, it's a while away though. 10-15 min walk down to Picadilly Gardens, there's a bus station there. Get a bus like the 42, nearly all buses leaving that station will go near the eye hospital but just ask if they go down Oxford Road before you get on.
If you get to Oxford Road train station, you still need to get that same bus down, but you can get it from Oxford Road. YOu could even walk, I think it's about 15 mins.
There are something ridiculous like 6 buses a minute, so don't worry about that.