In a letter published in The Lancet of Feb. 27th, p. 611, Dr. E. Magennis, writing of the conduct of barristers in Ireland, deplored the disappearance of that courteous treatment of the medical witness which once characterised cross-examination but which at the same time did not prevent the most vigorous investigation of the facts, and he drew attention to the unwarranted impertinence, frequently amounting to positive insult, which appears to arise from the assumption that the medical witness must not only be prejudiced but ready to give perjured evidence on behalf of the party employing him. There are many who agree with Dr. Magennis, and who will add that the discourteous treatment of the medical witness is not altogether confined to the Irish courts, although they may admit that such conduct is comparatively rare in England and that in either country it is not that of advocates of high standing and repute. Others, again, are ready to call attention to the contradictions of medical witnesses and to regret that in some instances these should be of a character to bring discredit to the medical profession. In this case, however, as in the others, the man who deserves blame is the exception rather than the rule; he is not one of those who should be taken as types of an honourable profession. The medical profession is at a disadvantage when we compare it with others which are called upon to supply expert evidence in courts of law, and this in more respects than one. Expert evidence is not evidence of fact, it is evidence of opinion based upon scientific knowledge. Medical men give evidence of fact when they speak in the witness-box as to physical conditions which have come under their actual observation; when they draw inferences from those conditions as to the duration of injuries, as to the amount of incapacity produced, or as to the cause of them so far as these points lie outside the limits of their knowledge, they then become expert witnesses. In the same capacity, for the most part they testify as to the mental condition of persons of doubtful sanity. These are examples of occasions upon which medical witnesses express opinions which they are qualified to give but which may well differ from those of other equally well-qualified persons who have looked at the same facts from a different point of view or who, holding different opinions upon doubtful topics, have a perfect right to express them. All such witnesses are entitled to be treated by members of the bar with the respect due from one learned profession to another.... The disadvantage, however, which particularly affects the medical profession is that its evidence is the expert testimony which is most often required. We have referred to some of the commoner kinds of expert evidence other than that of medical practitioners, but the engineers, the experts in art, the graphologists, and the chemists, who give evidence during any year, can hardly approach in number the medical witnesses who have to give evidence in cases of all kinds and to submit to cross-examination. Hence it has happened no doubt that the suggestion of readiness to espouse a side has been more often levelled against the medical witness than against others. All evidence of opinion is liable to be contradicted by the holders of opposite views upon the same scientific subject. The differences of human opinion have been proverbial from very early times in the history of the human race. In the law courts they are most commonly observable among medical men, and in the law courts the temptation is always present to the advocate to discredit the evidence against him by all means in his power, even by the imputation of corrupt motives by word or manner. It is not, however, the duty of the advocate to win the cause of his client per fas atque per nefas. As Sir Alexander Cockburn avowed upon a well-known occasion, amidst the cheers of his brother lawyers, the arms which he wields he should use as a warrior, not as an assassin. It rests with the judges, and with the honourable men in their profession in Ireland and elsewhere, to see that none among them gives occasion for such criticism as that of Dr. Magennis to which we have referred.