Abstract
When we put a thin section of an optically anisotropic crystal on the stag-e of the petrographic microscope, and observe between crossed nicols the intens-ity of light from the microscope, rotating the stage, we generally see that the intensity varies in accordance with the following equation theoretically obtain-ed from the FRESNEL'S law or the MAXWELL'S equations: [numerical formula] where A is the amplitude of the incident light, P a constant peculiar to both th-e thin section and the microscope, and θ the angle of rotation of the stage., With i-nterval of π/2 of θ, ω becomes 0, in other words, extinction occurs., While i-t is known that the (010) section of certain alkali-amphibole does not show ext-inction between crossed nicols, but the extinction occurs when the lower and up-per nicols are at a certain angle., The writer tried to obtain an equation which will represent quantitatively this abnormal optical phenomenon., The following e-quation give the intensity of light from the microscope in this abnormal case., This has been obtained on the basis of the assumption that, "when light enters the (010) section of the mineral, it is decomposed into two sets of plane polari-zed lights, and one of the each set is completely absorbed, the residual two po-larized lights vibrating not perpendicularly to each other., " [numerical formula] where P0, PI, and PII are constants peculiar respectively to the microscope and to two component lights in the mineral; χ the angle between the nicols; 〓 the angle between the planes of polarization of the component lights in the min-eral; and Δ the phase difference of the component lights., cos Δ is then the s-o-called retardation of the thin section., Measuring the intensity of the that this equation represents the abnormal extinction phenomenon as well as the variat-ion of the intensity of light.,
