Like a good many other convictions and persuasions that exist more or less as {4} commonplaces in the subconscious intellects of a great many people, this is not true. Our American humorist said that it is not so much the ignorance of mankind that makes him ridiculous as the knowing so many things "that ain't so." The supposed opposition between science and religion is precisely an apposite type of one of the things "that ain't so." It is so firmly fixed as a rule, however, that many people have accepted it without being quite conscious of the fact that it exists as one of the elements influencing many of their judgments--a very important factor in their apperception. Now, it so happens that a number of prominent original investigators in modern science were not only thoroughly orthodox in their religious beliefs, but were even faithful clergymen and guiding spirits for others in the path of Christianity. The names of those who are included in the present volume is the best proof of this. The series of sketches was written at various times, and yet there was a central thought guiding the selection of the various scientific workers. Most of them lived at about the time when, according to an unfortunate tradition that has been very generally accepted, the Church dominated human thinking so tyrannously as practically to preclude all notion of original investigation in any line of thought, but especially in matters relating to physical science. Most of the men whose lives are sketched lived during the fifteenth, sixteenth, and first half of the {5} seventeenth centuries. All of them were Catholic clergymen of high standing, and none of them suffered anything like persecution for his opinions; all remained faithful adherents of the Church through long lives. It is hoped that this volume, without being in any sense controversial, may tend to throw light on many points that have been the subject of controversy; and by showing how absolutely free these great clergymen-scientists were to pursue their investigations in science, it may serve to demonstrate how utterly unfounded is the prejudice that would declare that the ecclesiastical authorities of these particular centuries were united in their opposition to scientific advance.