I thought the quotation was something like "Justice must not only be done, but be seen to be done", but I was unable to find it in Bartlett's Quotations. I take it to mean that the law must be applied openly by the courts, not in camera, and that that is why there is a public gallery in a court.
The following is a quotation from Lord Justice Hewart, speaking at a trial in November 1923: "A long line of cases shows that it is not merely of some importance, but it is of fundamental importance, that justice should not only be done, but should manifestly and undoubtedly be seen to be done."