how did keeping the hat on helped evan with his escape plan

Evans did not take off his hat when Jackson asked him to do so because he reasoned that it was the only thing he had ever brought with him as a sort of lucky charm for his examination. However, the real reason was that he wanted to cut his hair and hide it while making his escape. It was bad luck when Jackson pinched his scissors because it had meant a long and tricky operation with the only razor blade he possessed. If Jackson had made him take off his bobble hat then the game would have been up. It was necessary to resemble McLeery in facilitating his plan of escape which would not have been possible without convincing the police of being so.