volume will not be found superfluous, particularly as it is founded on an independent study of Tycho's bulky works. To these I have given full references for every subject, so that any reader may find further particulars for himself without a laborious search. Many details, especially as to the historical sequence of Tycho's researches, have been taken from his original MS. observations in the Royal Library at Copenhagen, which I was enabled to examine during two visits to Copenhagen in 1888 and 1889. On the same occasions I also studied three astrological MSS. of Tycho's, of which an account will be found in Chapter VI. It may possibly be thought by some readers that I have devoted too much space to the consideration of the astrological fancies of the Middle Ages. But my object throughout has been to give a faithful picture of the science of the sixteenth century, and for this purpose it is impossible to gloss over or shut our eyes to the errors of the time, just as it would be absurd, when writing the scientific history of other periods, to keep silence as to the phlogistic theory of combustion, the emission theory of light, or the idea of the sun as having a solid nucleus. If the study of the history of science is to teach us anything, we must make ourselves acquainted with the by-paths and blind alleys into which our forefathers strayed in their search for truth, as well as with the tracks by which they advanced science to the position in which our own time finds it.
With the exception of the astronomical manuscripts in the Royal Library at Copenhagen (for facilities in using which I was indebted to Dr. Bruun, chief librarian), I have not made use of any unpublished materials; but the scanty harvest reaped by modern searchers makes it extremely unlikely that anything of importance remains to be found
