. On microscopical manipulation : being the subject matter of a course of lectures delivered before the Quekett Microscopical Club, January-April, 1869. the usual law {a7ite, p. 5); the other half will be refractedin a different direction. And if these two rays of lightbe examined with an analyser, they will be found, asin the case of the refledted light, to have acquiredthe property of sides, or to be polarised. The rayrefracted in the common way is known as the or-dinary ray; the other, which is polarised in a planeat right angles to it, is termed the extraordinary a crystal of Icelan
. On microscopical manipulation : being the subject matter of a course of lectures delivered before the Quekett Microscopical Club, January-April, 1869. the usual law {a7ite, p. 5); the other half will be refractedin a different direction. And if these two rays of lightbe examined with an analyser, they will be found, asin the case of the refledted light, to have acquiredthe property of sides, or to be polarised. The rayrefracted in the common way is known as the or-dinary ray; the other, which is polarised in a planeat right angles to it, is termed the extraordinary a crystal of Iceland spar be placed on a sheet ofpaper having a black spot on it, the double refradtion NICOLS PRISM AND TOURMALINE. 127 will cause the formation of two images of the spotor other device drawn on the paper. By an ingenious process, invented by Mr. Nicol, acrystal of Iceland spar is divided in the directionindicated in Fig. 35, and joined together again withFig. 35. f^\. Canada balsam. This causes one of the rays to beso much refra(5ted that it is thrown altogether out ofview, and only one polarised ray is transmitted. Thiscontrivance is known as the single image or Nicolprism, and is the means of polarisation most generallyadopted for microscopical purposes. Polarisation may also be effected by the use ofthin slices of a mineral knovv^n as tourmaline. Thismineral, when cut into thin plates parallel to the axisof the crystal, allows only the undulations in oneplane to pass, the other being stopped. The cause ofthis action is, as yet, not understood. If two plates oftourmaline are superposed, and held before a stronglight, it will be found that the light is alternatelytransmitted and stopped at every go°, when one ofthe plates is rotated. The same effec^t is produced, in a much greaterdegree, by using a pair of crystals of sulphate of iodo-quinine, more commonly known as Herapathite,from its discoverer, the late Dr. William Bird Hera-path. This salt i
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