66. It appears, doubtless, a vain idea to you that an end should ever be put to discovery; but remember, such impossibility merely signifies that mortal science must remain imperfect. Nevertheless, in many directions, the limit to practically useful discovery is rapidly being approached; and you, as students, would do well to suppose that it has been already attained. To take the science of ornithology, for instance: I suppose you would have very little hope of shooting a bird in England, which should be strange to any master of the science, or of shooting one anywhere, which would not fall under some species already described. And although at the risk of life, and by the devotion of many years to observation, some of you might hope to bring home to our museum a titmouse with a spot on its tail which had never before been seen, I strongly advise you not to allow your studies to be disturbed by so dazzling a hope, nor your life exclusively devoted even to so important an object. In astronomy, the fields of the sky have not yet, indeed, been ransacked by the most costly instruments; and it may be in store for some of you to announce the existence, or even to analyse the materials, of some luminous point which may be seen two or three times in the course of a century, by any one who will journey to India for the purpose; and, when there, is favoured by the weather. But, for all practical purposes, the stars already named and numbered are as many as we require to hear of; and if you thoroughly know the visible motions, and clearly conceive the known relations, even of those which can be seen by the naked eye, you will have as much astronomy as is necessary, either for the occupation of thought or the direction of navigation. 67. But, if you were discontented with the limit I proposed for your sciences, much more, I imagine, you were doubtful of the ranks I assigned to them. It is not, I know, in your modern system, the general practice to put chemistry, the science of atoms, lowest, and theology, the science of Deity, highest: nay, many of us have ceased to think of theology as a science at all, but rather as a speculative pursuit, in subject, separate from science; and in temper, opposed to her.