Light; a course of experimental optics, chiefly with the lantern . plates, G, made of the thinnest and most colourlessglass procurable. Ten or twelve are sufiicient, but the back require thirty hours of unremitting attention over a fire, as it is .impos-sible to leave the task when once commenced, till the balsam is bakedthoroughly hard. XI.] REFLECTING POLARISCOPE. 253 one must be blacked on the back, and the whole so arrangedthat the light from the condensers impinges upon and leavesthe glass at an angle of about 56° from the normal. Such anelbow and bundle, with the objective screwed in, an
Light; a course of experimental optics, chiefly with the lantern . plates, G, made of the thinnest and most colourlessglass procurable. Ten or twelve are sufiicient, but the back require thirty hours of unremitting attention over a fire, as it is .impos-sible to leave the task when once commenced, till the balsam is bakedthoroughly hard. XI.] REFLECTING POLARISCOPE. 253 one must be blacked on the back, and the whole so arrangedthat the light from the condensers impinges upon and leavesthe glass at an angle of about 56° from the normal. Such anelbow and bundle, with the objective screwed in, and a Nicolfitted on the nozzle, is the ordinary lantern gives a large field of polarised light, and for nearly allpurposes the reflector is almost as good as a large Nicol;but it has the two disadvantages, that the lantern has to beturned off sideways, on account of the elbow angle, andthat the original plane of polarisation cannot be,rotated,which is desirable for some experiments. Such experi-ments are, however, very few. To meet this objection,. Fig, T54,—Elbow of Polariscope. some have employed a glass pile, working by the trans-mitted light, which can be mounted and rotated just likea Nicol. This meets the objection just stated, but hasanother—viz., that it is difiicult to get complete polarisationin this way. If a refracting bundle is employed, not lessthan eighteen plates should be used, and the a,ngle shouldbe somewhat greater than the polarising angle, by whichnearly all unpolarised light may be reflected and so got ridof The greatest fault of such a pile so used, however, isthat it usually gives a perceptible green colour, owing to thethickness of glass. The ten or twelve plates often men-tioned, do not polarise the whole beam by a great deal. A 254 LIGHT. [chap. reflecting pile yields about 20 per cent. less light than aNicol of equal field. It is well to have an additional glass analyser, (Fig. iSSj)formed of eighteen or twenty pieces of microscop
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