Cokin Circular Polariser P164. Increases the colour saturation of blue skies, removes the unwanted reflections from non-metallic surfaces such as glass, water, etc... Use at a right angle (90°) between the shooting axis and the position of the sun and turn the filter in the holder to adjust the polarising effect.
 
Filter holder and adapter ring are sold separately. 
 
All Cokin filters are manufactured from CR-39 organic glass which was originally launched for the vision lens industry. 

CR-39 organic glass boasts several major features:
 
Extra lightweight
Highly resistant to shocks, meaning that photographers, videographers and cameramen can handle the filters in complete confidence and safety
Excellent optical transmission and high compatibility to colouring, making Cokin filters the proven choice of photographers worldwide

The Cokin Creative Filter System consists of three main components: A filter holder / Adapter ring and a Filter.

 Screw the adapter ring onto your lens.

Slide the filter holder on the adapter ring until it snaps in place. The filter holder is now solidly attached to the lens, yet it can rotate both left and right.

Slide the filter into one of the filter holder slots. The one closest to the filter holder housing is the narrowest; it allows the use of round filters with notched edges (polarizer, star filters..). The two central slots are intended for square filters. The outer slot can be used either for an additional filter or for a coupling ring.