Microscope observations for the studied chiral dimers in
glass cells with planar anchoring revealed that in both nematic
phases the optical axis was aligned perpendicular to the glass
substrates, and the transition to the NTB* was marked by
a sudden disappearance of optical activity. The conoscopic
image (Figure 3) clearly shows that the NTB* phase is uniaxial
with the positive optical anisotropy, contrary to the N* phase
having the negative optical anisotropy. Because in the NTB*
phase the refractive index is larger along the helical axis, the
conical angle of the helical structure must be less than magic