American horological journal, devoted to practical horology (1869)
Miller, George B · New York, G. B. Miller · leaf 268
and all bodies having unequal tension,whether from inequality of temperature orpressure. These bodies possess the property ofdoubling the image, or apparent position ol theobject, when the ray is transmitted in certaindirections. The reason of this is, that the pencil or rayof natural light does… … The cut represents a rhombohedron crystalof the Iceland spar, one of the many hundredforms of carbonate of lime; and perhaps wecould have taken no fairer example, for thedivergence of the two pencils is seen in aneminent degree. To test this quality, orrather to measure the amount of double…
Clocks and watches
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