. Compendium of meteorology. Meteorology. POLARIZATION OF SKYLIGHT 81 much simpler. Their elevation above the horizon is measured by standard procedures (usually by a pen- dukmi quadrant or a theodolite); for finding the neutral point any type of polariscope can be used. The most convenient type is the Sa\'art polariscope and its modi- fications. The main part of the polariscope is Savart's double plate (two plates of the same thickness cut under 45° from, the optical axis of a quartz or any crystal; one of them turned through a right angle from the other). If a polarized light passi
. Compendium of meteorology. Meteorology. POLARIZATION OF SKYLIGHT 81 much simpler. Their elevation above the horizon is measured by standard procedures (usually by a pen- dukmi quadrant or a theodolite); for finding the neutral point any type of polariscope can be used. The most convenient type is the Sa\'art polariscope and its modi- fications. The main part of the polariscope is Savart's double plate (two plates of the same thickness cut under 45° from, the optical axis of a quartz or any crystal; one of them turned through a right angle from the other). If a polarized light passing through the double plate is observed through an ana- lyzer with the plane of polarization bisecting the angle between the principal planes of transmittance of the plate, parallel color fringes appear. They have a dark or bright central band, depending on whether the in- cident light is polarized at right angles or parallel to the plane of of the double plate. The fringes disappear if the incident light is unpolarized. The modifications of Savart's polariscope differ with the of analyzer used. In the original model a tourmaline plate was used. Its great disadvantage was a strong absorption resulting in a dark green color of the field. By vising a Nicol or similar prism this dis- advantage can be removed, but the field is then verj^ small. Much larger fields and an extraordinary bright- ness of fringes can be reached in Voss's modification [69] with a Wollaston prism as analyzer. By a suitable ad- justment of the thickness of the double plate and the Wollaston prism, the deviation of the ordinary and extraordinary rays emerging from the prism can be made exactly equal to the angular distance of the in- terference fringes. In this way the fringes in the ordi- nary and extraordinary system of rays coincide and their intensity is doubled. Because of its great lumi- nosity the Voss polariscope is very useful for measur- ing neutral points late after sunset. I
Size: 1697px × 1472px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookcollectionbiodivers, booksubjectmeteorology
