What happened when the constable came to arrest the scientist?

The constable, Mr Jaffers was quite surprised to find that he had to arrest a man without a head. But, Mr Jaffers never gave up so easily. To him, if a magistrate’s warrant ordered a person’s arrest, then that person had to be arrested, with or without his head.  
He tried to get hold of a man who was becoming more and more invisible as he threw off one garment after another. When Griffin had taken off all his clothes, the constable found himself struggling with someone he could not see at all. The constable was knocked unconscious in the end.