An optical system for determining an optical constant by measuring the absolute
reflectance and the absolute transmittance of a substance by using an incoming
side beam switching mirror for selectively switching the direction of a light from
a light source to first or second converged light reflecting units. The first and
second converged light reflecting units project the light from the beam switching
mirror so as to be converged in an intersecting manner at the position of a sample
holder that can be positioned to present a sample fitting hole or a through hole
for measuring the reflectance/transmittance by providing the light to an exit side
beam switching mirror and detector.
