May 24, 1918] 



. SCIENCE 



509 



difficulty in securing first class spar from the 

 Iceland deposit has steadily increased, much 

 of the material taken out being useless for 

 optical purposes. 



Occasional finds of doubly refracting spar 

 have been made elsewhere without resulting 

 in the development of a new supply. 



While the total amount of spar required is 

 not great, the maintenance of a certain pro- 

 duction is necessary for the manufacture of 

 Nicols prisms to be used in dichroscopes for 

 testing pleochroism of gem stones, polariscopes, 

 polarizing microscopes and saccharimeters. 

 Other substances having as great a difference 

 in the refractive indices of the ordinary and 

 extraordinary rays are all unsuitable for re- 

 placing Iceland spar in optical instruments. 



It is possible that the Montana veins might 

 be made to return a commercial product of 

 spar if they were worked with sufficient care. 

 From one vein six hundred pounds of crystals 

 are said to have been shipped to an agent 

 who sold the spar in Germany, receiving 

 $3,000 therefor. These crystals had been 

 sorted from thirty to forty tons of calcite 

 blasted out in the sinking of a seventy-five- 

 foot shaft. 



The Montana deposit lies in two vertical 

 veins in gneiss. The veins are four to seven 

 feet wide, probably at least 100 feet deep, and 

 are several miles long. The deposits are near 

 the surface, easily rained, and quite accessible 

 to the railroad. The crystals practically fill 

 the entire vein without any admixture of for- 

 eign intrusions. 



So far, no absolutely perfect crystals have 

 been obtained from these veins, but it is al- 

 together likely that a better product may be 

 had by more careful mining. The imperfec- 

 tions are of two kinds. Some crystals have a 

 very slight, gray cloudiness, which renders 

 them unsuitable for optical purposes. This 

 defect is inherent. The bulk of the material, 

 however, while perfectly clear shows slight 

 cleavage in the interior of the crystals. This 

 may be and probably is due to the shocks to 

 which the crystals are subjected in mining. 



Some of the crystals were obtained by the 

 Bureau of Mines and submitted to the Bureau 



of Standards for test. Their report is quoted 

 below. 



The larger sample does show interference colors 



in places in its body as noted by . We 



are not, however, of the opinion that this renders 

 the whole crystal useless for optical purposes. It 

 would appear that good material for small optical 

 parts (e. g., small Nicols prisms) might be cut 

 from this crystal. 



It is also true that the smaller sample is very 

 slightly turbid (milky). This makes it not 

 strictly first class, but for some purposes would 

 not impair its use. Otherwise it is an exquisite 

 sample. We would like to have for our own use a 

 considerable supply of material. 



If a market could be developed for pure 

 calcite to pay for mining a large tonnage of 

 these deposits, it appears altogether probable 

 that good optical crystals could be obtained 

 as a by-product in quantity sufficient for all 

 scientific requirements, and so meet the need 

 caused by the diminishing output from Ice- 

 land. 



Chas. L. Parsons 



Bureau of Mines, 

 Washington 



SCIENTIFIC EVENTS 



INVENTIONS SECTION OF THE GENERAL 

 STAFF OF THE DEPARTMENT OF WAR 



The following statement is authorized by 

 the War Department : 



In order to secure prompt and thorough in- 

 vestigation of inventions submitted to the War 

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 All inventions of a mechanical, electrical, or 

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 whose investigations determine whether the in- 

 ventions have merit. Those with merit are 

 referred to the Advisory Board, which deter- 

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