Note 100 — Chapter VII

Bartholin, On Iceland Crystals: Copenhagen, 1669. An Accompt of sundry Experiments made and communicated by that Learn’d Mathematician Dr. Erasmus Bartholin, upon a Chrystal like Body sent to him out of Island: in connection with which Dr. Matthias Paissenius writes:—The observations of the excellent Bartholin upon the Island Chrystal are, indeed, considerable, as well as painful. We have here, also, made some tryals of it upon a piece he presented me with, which confirm his observations. Mean time he found it somewhat scissile and reducible by a knife into thin laminas or plates, which, when single, shew’d the object single, but laid upon one another shew’d it double; the two images appearing the more distant from one another, the greater the number was of those thin plates laid on one another. With submission to better judgements I think it to be a kind of Selenites. Some of our curious men here were of opinion that the Rhomboid figure proper to this stone was the cause of the appearances doubled thereby. But having tryed whether in other transparent bodies of the like figure the like would happen, we found no such thing in them, which made us suspect some peculiarity in the very Body of the stone.—Phil. Trans. for 1670, vol. v.