5cm Circle/ring prism to place in front of your lens to shoot through.  
This circular type of glass prism can give you the most spectacular effects by holding or hanging it in front of your lens while you take photos! 
 
It has a small hole you can loop some nylon thread or wire through to hold onto and keep your fingers out of the way. 
Take care not to scratch the front element of your lens but enjoy playing and seeing what kind of effects you can create.  See here for some examples.
 
We'd love to see what you create with these, tag us using #TDLprism so we can see!