"The very title is inaccurate.

_The Prison Walls must Cast no Shadow_." He repeated the sentence with a certain unction.
"The rhythm is perhaps not amiss but the metaphor is untrue.

My son pointed it out to me.

As he says, all walls cast shadows." "Yes," said Thresk.

"The trouble is to know where and on whom the shadow is going to fall." Mr.Hazlewood was startled by the careless words.