What does this mean? In the first place it shows, what all instructed persons know, that the Royal Society did then exercise the privilege of giving an _imprimatur_ at any rate to books written by its own Fellows. It cannot be supposed that such _imprimatur_ guaranteed the accuracy of all the statements made by Tyson, for we may feel sure that John Hoskins was quite unable to give any such assurance. We must assume that it meant that there was nothing in the book which would reflect discredit upon the Society of which Tyson was a Fellow and from which the _imprimatur_ was obtained. However this may be, the sway over its Fellows' publications was exercised, and indeed very excellent arguments might be adduced for the reassumption of such a sway even to-day.[26] Though the _imprimatur_ in question has fallen into desuetude, it is, as we all know, the commonest of things for the introductions to works of science to occupy some often considerable part of their space with acknowledgments of assistance given by learned friends who have read the manuscript or the proofs and made suggestions with the object of improving the book or adding to its accuracy. Any person who has written a book can feel nothing but gratitude towards those who have helped him to avoid the errors and slips to which even the most careful are subject. So that such acknowledgments of assistance have come to be almost what the lawyers call "common form." What they really amount to is a proclamation on the part of the author that he has done his best to ensure that his book is free from mistakes. Now the _imprimatur_ really amounts to the same thing, for it is, of course, confined to books or parts of books where theology or philosophy trenching upon theology is concerned. Thus a book may deal largely, perhaps mainly, with scientific points, yet necessarily include allusions to theological dogmas. The _imprimatur_ to such a book would relate solely and entirely to the theological parts, just as the advice of an architectural authority on a point connected with that subject in a work in which it was mentioned only in an incidental manner, would refer to that point, and to nothing else. Perhaps it should be added, that no author is obliged to obtain an _imprimatur_ any more than he is compelled to seek advice on any other point in connection with his book. "_Nihil Obstat_," says the skilled referee: "I see no reason to suppose that there is anything in all this which contravenes theological principles." To which the authority appealed to adds "_imprimatur_:" "Then by all means let it be printed." The procedure is no doubt somewhat more stately and formal than the modern system of acknowledgments, yet in actual practice there is but little to differentiate the two methods of ensuring, so far as is possible, that the work is free from mistakes. That neither the assistance of friends nor the _imprimatur_ of authorities is infallible