Transactions . icles of gravel sticking to the walls, which then presenta covering, or coat, which in the sunshine has a beautiful andglittering effect. Many years ago, the writer knew a dwelling-house at Westgate, Weardale, called Sparry Hall, the wallsof which were treated in this manner. Sometimes the gravel issubjected to a general cement-wash, which gives the walls agrey and rough appearance. 222 OCCURRENCE AND COMMERCIAL USES OF FLUORSPAR. Fluorspar in small quantities was dispatched from Weardalefor use as a flux so early as the year 1847, and large quantitieswere, twenty years ago, sen
Transactions . icles of gravel sticking to the walls, which then presenta covering, or coat, which in the sunshine has a beautiful andglittering effect. Many years ago, the writer knew a dwelling-house at Westgate, Weardale, called Sparry Hall, the wallsof which were treated in this manner. Sometimes the gravel issubjected to a general cement-wash, which gives the walls agrey and rough appearance. 222 OCCURRENCE AND COMMERCIAL USES OF FLUORSPAR. Fluorspar in small quantities was dispatched from Weardalefor use as a flux so early as the year 1847, and large quantitieswere, twenty years ago, sent to Scotland and America. Fig. 4shows a heap of the mineral lying at Wearhead station. Optical Properties.—It has been stated that transparent fluor-ite was sometimes used, on account of its optical properties, inthe manufacture of microscope objectives and other refinedoptical instruments, but the writer was quite recently informedby an optician that he did not think fluorspar was ever used forthis Fig. 4.—Fluorspar (from the Sedling Mine) at Wearhead Station. Mr. F. W. Rudler, however, referring to fluorspar destituteof colour, states that the dispersive power and refractive powerof fluor render the pellucid colourless mineral of value forthe construction of objectives for microscopes, as introduced byProf. Abbe. A variety from Switzerland has been applied tothis purpose. The lenses of fluor are used in conjunctionwith lenses of Jena glass, so as to produce an improved kindof achromatic objective which is termed apochromatic * A Handbook to a Collection of the Minerals of the British Islands, mostlyselected from the Ludtam Collection, in the Museum of Practical Geology, by W. Rudler, 1905, page 87 ; and Jena Glass, by Dr. H. Hovestadt, translatedand edited by Prof. J. D. Everett and Mrs. Alice Everett, 1902, page 88. OCCURREXCE AND COMMERCIAL USES OF FLUORSPAR. 223 Fluorspar as a Flu .v.—We are informed by Prof. Vivian that fluorspar has been us
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