Two essays on the nature and wonderful properties of Island Crystal : Essay I. Containing, all that has been hitherto delivered in regard to its form, dimensions, and double refraction of light; with the addition of several new experiments with polished parallelopipeds and prisms made of this substance, which discover a various, multiple, and colorific refraction; and exhibit a variety of images of one and the same object. To which is subjoined, a computation of the magnifying power of reflecting telescopes; and a table thereof, from 6 inches to 12 feet in length. The whole illustrated with copper plate figures. By Benjamin Martin.