the original position, viz., reflected rays red, refracted green. The thickness of the films of selenite determines the particular colour produced. [Illustration: Fig. 334. The electric lamp and lantern of Duboscq, showing the projection of the carbon poles on the disc. This experiment is performed with the help of the plano-convex lens, A, and the rays pass through a very narrow aperture at B.] [Illustration: Fig. 335. A A. Card model of a beam of polarized light coming from the first bundle of plates of glass, shown at Fig. 326, p. 339. B. Model of the film of selenite, which divides or dipolarizes the ray A A into C and D, which, interfering by means of the second bundle of plates of glass called the analyser Z, produce reflected chromatic effects by interference at E, and refracted effects at F.] If the selenite is of a uniform thickness, one colour only is obtained, and by ingeniously connecting pieces of various thicknesses (in the same forms as stained glass for cathedral windows), the most beautiful designs were made by the late Mr. J. T. Cooper, jun., which have since been manufactured in great quantity and variety by Mr. Darker, of Paradise-street, Lambeth. The colours of these selenite objects are seen by placing them in front of a piece of black glass, fixed at the polarizing angle, and then examining the design with a slice of tourmaline, or still better with a single-image Nicol prism, when the most brilliant colours are obtained, and varied at every change of the angle of the analyser. Selenite, or sparry-gypsum, is the native crystallized sulphate of lime, which contains water of crystallization (CaO, SO_{3}, 2H_{2}O). It frequently occurs imbedded in London clay, and is called _quarry glass_ by the labourers who find it at Shotover Hill, near Oxford, and also in the Isle of Sheppey. At a very early period, before the discovery of glass, selenite was used for windows; and we are told that in the time of Seneca, it was imported into Rome from Spain, Cyprus, Cappadocia, and even from Africa. It continued to be used for this purpose until the middle ages, for Albinus informs us, that in his time, the windows of the dome of Merseburg were of this mineral. The first greenhouses, those invented by Tiberius, were covered with selenite. According to Pliny, beehives were encased in selenite, in order that the bees might be seen at work. [Page 347] The late Dr. Pereira has placed the phenomena already described in the form of a most instructive diagram, which we borrow from his elaborate work on "Polarized Light." (Fig. 336.)