I have just purchased a long mirror & on hanging it (horizontally) i find It distorts the image fat & thin . How do i correct this please. I think it is either concave or convex
If it's very long, I'd suspect that it's being physically distorted during the installation process since the glass itself is flexible enough to distort the image without breaking first. To correct it, you need to make sure that the mirror is as straight as possible - use shims.
If it's along a wall, the mirror may be following the contours of a slightly uneven wall. You can shim the back of the mirror in the hollow spots to correct this. Masking tape (ideally the blue kind to protect the mirror) can be used in the low spots to build up sufficient support for the glass. Thicker shims can be used if you find the hollow spots to be very thick.
It's it's in a frame of some sort, either the frame is not straight, or it is sagging/distorting too, or (if the glass is sufficiently loose int he frame) the mirror itself is just distorting as above. The approach is basically the same - shim the spots that you need to push forward.