the course of time. A remedy for this was sought in complicated extensions of his teachings; this, however, only made matters so much worse, without explaining the facts. King Alfonso X of Castile is reported to have said to his astronomers that, if the arrangement of the universe had fallen to his lot, he would have made things much more simple. All of these difficulties were suddenly removed by Copernicus. The course of the planets now no longer appeared a cause for great perplexity, but admitted of a simple explanation and resolved itself into one grand harmony. Then, after Kepler had discovered the three laws that bear his name and which mark a new era in the science, the mathematical part of the work was brought to an end by Newton's discovery of the law of gravitation.
Eclipses of the sun and moon always attract general attention. On such occasions it is not only the phenomena themselves that call for our admiration, but mainly the art which makes possible the prediction of these events to the hour, the minute, ay, the second; by means of which one may know in advance whether the eclipse will be total, partial, or annular, what part of the sun or the moon will be first covered, how long the phenomenon will last, and from what parts of the world the eclipse will be visible.
If a prediction rest on a hypothesis prepared ad hoc, no matter how ingenious it may be, our doubts and our mistrust are justified. In such a case we have the right to ask for confirmation, and to demand that the hypothesis shall be extended into a theory by its sequence, and that this sequence shall stand in accordance with the actual facts. If, however, a prediction is based on a theory which approaches in thoroughness and in extensive confirmation the cases we have cited as examples, and furthermore, if the separate instance be deduced in a strictly logical manner, then the prediction is worthy of our confidence. As most systematic classifications are of value only in affording a general view of the ground, without being able to embrace all cases, so, too, in our classification, we meet with instances of transition and combination.
Of this, examples are to be found in all the sciences, but notably so in medicine, geology, and meteorology. The predictions in medicine to a great extent form transitions between the first and the second class; that is to say, they are based in part on Bacon's, in part on Mill's system of induction. Thus, of late, the theory of bacteria has come to be of great importance. This theory is of recent origin, but has deservedly many champions, and offers an insight into new ways, which medicine, in combination with natural science, must explore in order to obtain valuable results. Based on these views, which would designate certain bacteria as carriers of certain diseases, new precautionary measures have been adopted, which are to serve as a guard against the phantoms of disease; and in many places these measures have already proved of great value.
