Circular dichroism and circular birefringence are manifestations of optical activity. This is called birefringence or optical anisotropy. Some proteins are also birefringent, exhibiting form birefringence. Circular birefringence and circular dichroism are the manifestations of optical activity. Single calcite crystals display an optical property called birefringence (double refraction). Calculations also indicate birefringence and dichroism at high fields. The Lyot filter is a specialized narrowband spectral filter employing the wavelength dependence of birefringence. The property of birefringence (or double refraction) is observed in many optical crystals. In the simplest form, uniaxial birefringence, there is only one special direction in the material. Gelatin solutions show viscoelastic flow and streaming birefringence. It is anisotropic and has high birefringence. Under polarized light microscopy, they have a needle-like morphology and strong negative birefringence. Biotite has a high birefringence which can be partially masked by its deep intrinsic color. He then explained how optical rotation could be understood as a species of birefringence. Calcium pyrophosphate crystals, in contrast, show weak positive birefringence.