Hi Zac,
The Nagel is by far the most difficult test to pass and many colour normal persons fail to reach the JAA Pass Standard.
There is nothing you can do to prepare for it.
You look through an eyepiece at a split field circle and the objective is to match the top and bottom semi-circles.
Even when you've matched them, you only have to move your eye a fraction and the semi-circles look different, in exactly the same way as an image on a computer monitor looks different when you view it from different angles. So, unles you know exactly where your eye is supposed to be in the first place you are immediately disadvantaged.
Sorry to be the bearer of such negative information.
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