Another way to repair those broken specs is by using two needles to put back the two pieces.
This option is more discreet and the break will be harder to notice. It's the best choice when you broke your glasses in half.
Here are the steps:
Heat up the plastic frames.
Do not use an open flame as it will damage the plastic and you won't be able to repair them. Instead, boil some water, hold the frames over the hot steam. This way, you will be sure that they won't melt.
Now that the plastic is soft, flexible and compliant you can introduce a metal pin on each side.
Unite them as if you were completing a puzzle and push the edges towards one another. Because the plastic is soft, the metal pins will enter smoothly.
Let it chill until the plastic is once again hard.
Four stars because it's easy and it must be interesting puzzlin' up your frames.