For the (almost) final touch we will apply a luminosity mask, which is easiest done by pressing Ctrl+Alt+~ (the tilde on the key just above the Tab key) on US-keyboards on my non-US, Scandinavian keyboard I have to press Shift+Ctrl+Alt+´ (the key on the left to backspace). The other way is to jump to the Channels window and click on the "Load channel as selection" button (a dotted ring at the bottom of the window) or Ctrl-click on the RGB channel. Now we have selected all the lighter parts of the picture, but it is actually the darker parts we are going to enhance, so we invert the mask with Shift+Ctrl+I (or Select/Inverse).
With the selection active, make a new curves adjustment layer. The selection will now be added as a mask channel to the adjustment layer.
Now on the RGB-curve, add an anchor point at the 2-2co-ordinates. Add another point at 5-5 and pull it 1,5 steps to the left. Click OK and change the opacity to about 40.
