1911-1912.] Iceland Spar and the Polariscope. 443 



owner of the mine was one Carl Tulinius, a Dane, who became 

 possessor by marrying Erlendsson's daughter, who received the 

 mine as an inheritance. This man owned and worked the 

 mine almost every year from one to three months of each 

 year. As is well known, the climate in Iceland only permits 

 one to work for a very short season, after which every place 

 is covered with snow and ice. Gunpowder was first used for 

 blasting in 1880. In 1885 Carl Tulinius sold the mine to 

 the Icelandic Government. The finest piece of spar in exist- 

 ence is in the South Kensington Museum in London, and is 

 worth over £2000. It was mined out by Carl Tulinius's men. 

 This man was succeeded by his son Thore in 1898, who hired 

 and wrought the mine till 1906. It is now in the hands of a 

 French syndicate. Many stories might be told in connection 

 with this mine, — some tragic, some amusing. The first man 

 to make a study of Iceland spar was Erasmus Bartholinus, 

 who was born in 1625 and died in 1698, son of Gaspard 

 Bartholinus. Erasmus spent ten years visiting England, 

 Holland, Germany, and Italy, and filled the chairs of mathe- 

 matics and medicine in Copenhagen. He had received from 

 a Danish sailor who had been to Iceland a piece of spar, and 

 had discovered the double refraction. He wrote a treatise on 

 it in his * Experimenta Crystalli Islandica ' in 1669. 



"Iceland spar is composed of 56 per cent of lime and 44 

 per cent of carbonic acid, and is absolutely the purest form of 

 carbonate of lime. It is, when of good quality, very trans- 

 parent, and generally colourless. Its natural faces, when 

 split, are mostly even and perfectly polished, but when they 

 are not so, we can by a new cleavage replace the imperfect 

 face by a better one, or we may grind and polish any imper- 

 fect face. It does not matter whether the spar be found in 

 mass or crystal, we can always cleave it or split it into the 

 shape which is known by the name of a rhomb. A rhomb of 

 Iceland spar is a solid bounded by six equal sides and similar 

 rhomboidal surfaces. The axis of the rhomb is equally in- 

 clined to each of the six faces at an angle of 45° 23V — 

 (Brewster.) The probable cause of the double refraction is 

 owing to the mineral, whilst cooling, having had very unequal 

 pressure or unequal temperature. I do not here propose 

 going into the explanation of the double refraction, as the 



