Polariscopio di Savart

Descrizione e caratteristiche fisiche

Il polariscopio di Savart si compone di due quarzi obliqui, dello spessore di 4 o 5 millimetri, sovrapposti per modo che i due assi si taglino ad angolo retto. La luce polarizzata che attraversa questo sistema guardata per mezzo di una tormalina la cui sezione principale faccia un angolo semiretto con ciascuno dei due assi , mostra delle frange parallele vivamente colorate. Se la direzione di queste frange coincide col piano di polarizzazione si vede nel mezzo una frangia nera, che si trasforma in bianca quando la direzione delle frange diviene normale al piano di polarizzazione (v. figure a lato).

Lo strumento doveva essere provvisto di custodia e corredato di quattro filtri colorati intercambiabili (rosso, arancio, verde, blu), ora mancanti.

Savart's polariscope is a simple and delicate contrivance for the detection of plane-polarised light (Preston, 1890). A thin plate of a uniaxal crystal is divided into two halves in order to secure two similar thin plates of equal thickness. The two portions are now superposed so as to form a plate of double the thickness, and one of them is rotated through 90°, so that their principal sections are at right angles. The plates, so placed, are mounted in a small tube before Nicol's prism, or any other analyser, of which the principal section is turned parallel to the bisector of the angle between the principal sections of the plates. When a beam of convergent or divergent plane-polarised light is allowed to fall upon the plates the field of the analyser is crossed by a system of coloured rectilinear fringes parallel to the bisector of the angle between the principal planes of the plates. The sensibility of the instrument increases as the plane of polarisation of the incident light approaches the direction of the fringes, that is, the bisector of the angle between the principal sections, and it is sufficiently delicate to easily detect the polarisation of the light of the sky. The quartz plates are cut at an angle of about 45° to the axis in order to avoid the effects of the rotatory power which the crystal possesses in the direction of its axis.