Every student who enters upon a scientific pursuit, especially if at a somewhat advanced period of life, will find not only that he has much to learn, but much also to unlearn.
Almost all the greatest discoveries in astronomy have resulted from what we have elsewhere termed Residual Phenomena, of a qualitative or numerical kind, of such portions of the numerical or quantitative results of observation as remain outstanding and unaccounted for, after subducting and allowing for all that would result from the strict application of known principles.
First, In showing in how to avoid attempting impossibilities. Second, In securing us from important mistakes in attempting what is, in itself possible, by means either inadequate or actually opposed to the end in view. Thirdly, In enabling us to accomplish our ends in the easiest, shortest, most economical, and most effectual manner. Fourth, In inducing us to attempt, and enabling us to accomplish, object which, but for such knowledge, we should never have thought of understanding. On the ways that a knowledge of the order of nature can be of use.
