Dr. William Johnston was a physician and first Professor of Mathematics at Marischal College from 1626 until his death in 1640. He was married to a cousin, his mother’s niece Barbara Forbes, and had 3 children: one son, Thomas, who predeceased his father in 1636, and two daughters, Anna and Elizabeth. Dr. Johnston was proprietor of lands of Beideleston at Dyce. After his death, his 2 daughters shared division of his significant land holdings as heir proportioners of the estate. This was the law in Scotland: when there was no male heir, the estate was divided among the surviving daughters. This contrasts with the practice in England where estates passed to the nearest male relative, even be it a distant cousin.
Dr. Johnston, physician, is mentioned by Spalding in his Memorable Transactions, as having been sent in March 1639, along with George Morison, burgess in Aberdeen, as commissioner for that town to a meeting of the Covenanters held at Old Montrose[i].
Both mathematics and medicine were primitive in the early 1600s by today’s standards. Galileo had just recently proposed his revolutionary idea that the planets revolve around the sun. Calculus would not be invented by Leibniz and Newton for another 60 years. Medicine was dominated by superstition and ineffective herbal remedies. Even these latter were discouraged: women who worked with herbal potions were sometimes branded as witches. Anatomy was poorly understood: the church discourage dissection of human bodies, making knowledge difficult to come by. William Harvey published his revolutionary finding that the blood circulates through blood vessels in 1628. An English translation would wait another 20 years to appear.
After Dr. Johnson’s death in 1640, his widow Barbara donated his mathematics library and instruments to Marischal College, as recorded in a resolution of appreciation by the Aberdeen City Council[ii].
William Johnston was the son of George Johnston of that Ilk of Caskieben and his wife Christian Forbes, daughter of William 7th Lord Forbes[iii]. William’s wife Barbara Forbes was also descended from William 7th Lord Forbes through her father, Abraham Forbes, son of Lord Forbes[iv,v]. Thus were William and Barbara respectively 9th-great grandson and 9th-great-granddaughter of King Robert the Bruce of Scotland. The proof of these lineages is detailed in the biography of their daughter, Elizabeth Johnston.