"While it has often been emphasized, and we would repeat, that the decision in these cases, as in those in which insanity is pleaded, is for the jury and not for doctors, the verdict must be founded on evidence. If there are facts which would entitle a jury to reject or differ from the opinions of the medical men, this court would not, and indeed could not, disturb their verdict, but if the doctors' evidence is unchallenged and there is no other on this issue, a verdict contrary to their opinion would not be 'a true verdict in accordance with the evidence.' ... But we have to bear in mind that Parliament has altered the law and decreed that if a killing is committed by a person whose abnormal mind has seriously diminished his responsibility the verdict is to be manslaughter and not murder. If, then, there is unchallenged evidence that there is abnormality of mind and consequent substantial impairment of mental responsibility, and no facts or circumstances appear that can displace or throw doubt on that evidence, it seems to the court that we are bound to say that a verdict of murder is unsupported by the evidence. ... The Act directs the court to allow an appeal if they think that the verdict of the jury should be set aside on the ground that it is unreasonable or cannot be supported having regard to the evidence. ... If there is evidence and a proper direction, this court will not usurp the function of the jury, unless, indeed, there is evidence so overwhelming that the court comes to the conclusion that though it might be said there was some evidence the other way, the verdict would amount to a miscarriage of justice. ...