The dual dating of certain letters in this work, e.g. March 13th (23rd) 1675, reflects the differences in the dual calendrical system -- the Julian and Gregorian -- then employed in Europe. In the sixteenth century, papal authorities, out of a concern over the growing discrepancy between the calendrical accuracy of feast days and their actual celebration, acted to correct this discrepancy. The discrepancy was the result of the accumulated difference between the Julian calendar year, in use since the mid-first century, of 365.25 days and the correct yearly value of 365.242199 days. This resulted in a Papal bull of 1572 by Pope Gregory XIII directing the Jesuit astronomer Christopher Clavius ( 1537- 1612) to draw up a new calendar. This new formulation, known as the Gregorian calendar, was promulgated by Papal bull in 1582 and involved an omission of ten calendar days between October 4th and October 15th. However, in spite of the greater accuracy of the new calendar, religious divisions between the Roman Catholic, Eastern Orthodox and Protestant churches impeded the calendar's adoption. The Protestant lands were particularly slow and not until 1699-1700 did Denmark, the Dutch and German Protestant states adopt the Gregorian calendar. England and Sweden did not adopt it until 1752 and 1753, respectively. Consequently, references to letters written from these Protestant lands prior to the time of adoption of the Gregorian calendar frequently contain two dates: the originally assigned (Julian calendar) dating and, in parenthesis, the corrected (Gregorian calendar) dating.