Question

If, ... my father had taken the pains to explain to me that the principles of Agrippa had been entirely exploded and that ... the powers of [Agrippa] were chimerical, ... I should certainly have thrown Agrippa aside... A chimera is a deception or fantasy of the mind. Why does the author use the word chimerical here?
To show that everything in the world seemed imaginary

To show that the narrator was an old man at the end of life

To show the principles seemed true when they were not

To show the narrator has been haunted by his own past for years

Answer 1

I'd say the author used the word chimerical here to show the principles seemed true when they were not.
In the excerpt, the narrator is trying to say that he believed in Agrippa's teachings religiously, when in fact they probably were blown out of proportion and slightly exaggerated. Thus, his belief in Agrippa's science disappeared as he realized that nothing was true, at least not the way he believed it to be. 

Answer 2

Answer: To show the principles seemed true when they were not.

By saying that the powers of Agrippa were chimerical, the author means that those powers (the principles) were not what he expected to be, that they were a complete fantasy which he expected originally to be true, but then he learned that were not.