. The Bell System technical journal . a thin crystal can be examined by viewing through its edge or length. The carriage can be removed and raw crystals examined. The opticaxis is plainly visible and quite accurate orientations can be made if thereis not too much opaque material on the crystal. Excessive optical twinning 252 BELL SYSTEM TECHNICAL JOURNAL makes a confused pattern but good orientations can be made anyway. Araw crystal can be mounted adjustably in a jig that is lowered into theconoscope, the optic axis lined up, the jig transferred to a saw, and sectionssawed directly. Let us tur
. The Bell System technical journal . a thin crystal can be examined by viewing through its edge or length. The carriage can be removed and raw crystals examined. The opticaxis is plainly visible and quite accurate orientations can be made if thereis not too much opaque material on the crystal. Excessive optical twinning 252 BELL SYSTEM TECHNICAL JOURNAL makes a confused pattern but good orientations can be made anyway. Araw crystal can be mounted adjustably in a jig that is lowered into theconoscope, the optic axis lined up, the jig transferred to a saw, and sectionssawed directly. Let us turn now to the quantitative analysis of the ring pattern seen inthe eye piece when examining a uniaxial crystal. We wish to know the sizeof the smallest ring in the field, or rather the corresponding angle in thecrystal. This first dark ring (analyzer and polarizer crossed) is the resultof the slow wave falling one wave length behind the fast one. If the platethickness (Fig. ) is / the path length in the crystal is / = cos 6 (). Fig. —The angle of the smallest ringThis is to be substituted in Eq. , namely: iV = - (w/ — n,)A () Now it can be shown that, quite accurately, at the angle 9 from the opticaxis: tis - n/ = .00917 sin^^ () where .00917 is the difference in the refractive indices for the ordinary rayand the extraordinary ray for green mercury light traveling at right anglesto the optic axis. (These are generally given the symbols Ug and He or n^and Hf respectively.) i Ni = X .00917 sin 6 = 1 X cos 6 and since X = .000546 mm. for green mercury light this may be written/ sin etsine = mm. () SPECIFYING QUART/ CRYSTAL ORIENTATION whence we solve for the values in this table convergence ^ = 5° 10° 20° 30° 253 thickness / = This shows that if we wish to examine thin plates in a conoscope the lensesmust be strongly convergent. The conoscope used in the Western Electrichas a convergence corresponding to about the 20° entry
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