"'the word substantial must mean that the evidence concerned has something more than trivial probative value."
"It seems to us that the trial judge's task will be to evaluate the evidence of bad character which it is proposed to admit for the purpose of deciding whether it is reasonably capable of assisting a fair minded jury to reach a view whether the witness's evidence is, or is not, worthy of belief. Only then can it properly be said that the evidence is of substantial probative value ..."