Characters Observed with both Polarizer and Analyzer[46] in Position, that is with “Crossed Nicols.” When the nicols are accurately crossed, the field should be quite dark, and if this is not the case the adjustments must be looked to. The condensing lens should be removed for these tests as they are to be made with parallel light, but as a matter of convenience instead of removing the condensing lens, the polarizer with the lens on top may be lowered, when the results will be about the same as with parallel light. White light is supposed to be used. ▄Isotropic Character.▄ Sections of isotropic crystals are perfectly dark and remain so during a complete rotation of the stage through 360°. The explanation is very simple. Light being transmitted by an isotropic crystal in all directions without double refraction; it follows that the light from the polarizer, after having passed through the section, comes to the analyzer still vibrating in the plane of vibration of the polarizer. Hence it is entirely cut out by the analyzer. _Amorphous_ transparent substances act in the same way and remain dark during complete rotation of the stage. _Optical anomalies_, _i. e._, double refraction, may occur in isometric crystals and in amorphous substances that have been subjected to strains. ▄Anisotropic Character.▄ Sections of anisotropic crystals, having the property of _double refraction_, produce in general some _interference_ or _polarization color_, except as mentioned later. The popular explanation is as follows: [Illustration: FIG. 16. ] In Fig. 16, let PP′ be the plane of vibration of the polarizer, and A′A the plane of vibration of the analyzer. All the light, after it has passed through the polarizer, is vibrating parallel to PP′ when it reaches the lower side of the transparent crystal, _cdef_, on the stage of the microscope. In this transparent section let _ob_ and _oa_ be the two _directions_ of _vibration_, _i. e._, the only two directions parallel to which rays of light can vibrate in passing through the section. Let _om_ represent the amplitude of vibration of a ray from the polarizer. When this ray reaches the section it cannot get through it vibrating in the direction _om_, but is doubly refracted and of the two resulting rays, one gets through vibrating in the direction _ob_ and the other vibrating in the direction _oa_. From _m_ draw perpendiculars to _ob_ and _oa_. Then according to the law of the parallelogram of forces _ob_ will represent the amplitude of vibration of the ray passing through the crystal vibrating in the direction _ob_, and _oa_ will represent the amplitude of vibration of the ray passing through the crystal vibrating in the direction _oa_. We will thus have two rays passing through the crystal, polarized at right angles to each other. [Illustration: FIG. 17. ]