REPORT
(4), in 1915, described an appara tus designed to measure ellipticity and rotation; later, a simplified version of this apparatus (5) was used to measure the ellipticities of weakly dichroic samples. The lat ter device is represented in Figure 17 and operates as follows: in the absence of an active sample, the quarter-wave plate Q is removed and the polarizer Ρ and analyzer A are first set to extinction. A is a Lippich-double-field analyzer whose angular setting is precisely defined at extinction by a condition of equal illumination in the two halves of its field. Q is then in serted and rotated until its axis is parallel to the transmission axis of the polarizer and extinction is re stored. Q is mechanically mounted in such a manner as to allow its re moval and reinsertion without dis turbing its relative orientation. The optically active sample S is now added, Q is removed and Ρ ro tated to compensate for optical ro tation due to the sample and restore extinction with A. Q is then rein serted and A rotated to restore ex tinction again; the angle of rota tion of A is then equal to the ellip ticity produced by the sample, pro vided Q is exactly a quarter-wave
POLARIZER
Ν
SAMPLE
plate at the wavelength used for the measurement. If 8 is the phase re tardation due to Q, it may be shown that the measured and true elliptic ities, Θ' and Θ, are related by the following expression: θ' = θ sin δ
In practice, it may be necessary to use a number of quarter-wave plates to cover a wide range of wavelengths. Kuhn and Braun {22) described in 1930 an apparatus employing photographic methods for measur ing circular dichroism in the visible and ultraviolet spectral regions ; this apparatus is represented in Figure 18: light from an arc source il luminates slit Si ; lens Li images Si on S 2 ; Ρ is a split polarizer consist ing of two Rochon polarizers wedged and cemented with a small angle, 2 θ , between their trans mission axes. The two ordinary beams from Ρ are linearly polarized at -f- Θ and — θ relative to the plane of incidence at total internal reflection in a specially designed Fresnel water-rhomb F ; the beams exiting F exhibit equal elliptical polarizations of opposite sense and of ellipticities ± θ. In the absence of a dichroic sample, the analyzer
λ •JfPLA' ΓΕ
DOUBLE-FIELD ANALYZER {LIPPICHÎ
V\ " ~s~
Figure 17. Measurement of circular dichroism and optical hat's apparatus ( 1 9 3 0 )
--is I
(28)
SAMPLE
J ANALYZER
'
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rotation—Bru-
LENS | SPECTRO SCOPE SLIT PHOTOCRAPHIC PLATE
LEN
1(β)
J ^ Figure 18. Measurement of circular dichroism in the visible and u l t r a v i o l e t — Kuhn and Braun's apparatus ( 1 9 3 0 ) Circle No. 53 on Readers' Service Card
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