. On microscopical manipulation : being the subject matter of a course of lectures delivered before the Quekett Microscopical Club, January-April, 1869. osition of the the small prism, or analyser, be now put in itsplace, either over the objedl-glass or, better still,over the eye-piece, it will be found that the alternatetransmissions and stoppages of light take a film of selenite be now placed on the stage ofthe instrument, it will be found that the light will becoloured ; and, when one of the prisms is rotated 90°,the colour will change to the complementary instanc
. On microscopical manipulation : being the subject matter of a course of lectures delivered before the Quekett Microscopical Club, January-April, 1869. osition of the the small prism, or analyser, be now put in itsplace, either over the objedl-glass or, better still,over the eye-piece, it will be found that the alternatetransmissions and stoppages of light take a film of selenite be now placed on the stage ofthe instrument, it will be found that the light will becoloured ; and, when one of the prisms is rotated 90°,the colour will change to the complementary instance, a film that transmits red light in oneposition of the prisms will transmit a green of suchquality as to produce white light, were the twomixed. Blue would produce orange every quarterrevolution; and so on through the other tints of thespecftrum. Similar effedts are produced by using 130 MICROSCOPICAL MANIPULATION. films of mica, and all crystals possessing the propertyof double refradtion. To understand these changes of colour, it will benecessary to describe the dire(5tion of the axes of acrystal of selenite, which is represented in Fig. 37,. A, D and c, B being the position of the depolarisingaxes, and e, f and c, h that of the optic axes. Tounderstand the powers of these regions of the crystal,it will be necessary to try another experiment, which,although more advantageously shown with a largepolariscope, can be done sufficiently well under themicroscope. Let the film of selenite be so mountedas to be capable of rotation. If the stage of themicroscope is capable of concentric rotation, nofurther contrivance is necessary. If the prisms nowbe arranged for the production of colour, and theselenite slowly rotated, it will be observed thatthe colour gradually disappears. On continuing therotation, it increases, and regains its intensity. Ifthe amount of rotation is noticed, ibwill be foundthat the disappearances coincide with the positionof the optic axes, e, f and g, h, whil
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Keywords: ., bookcentury1800, bookdecade1870, booksubjectmicroscopy, bookyear1
