In order to achieve highly dispersive spectrometry in wide wavelength with the
use of a single grism, but not a plurality of grisms, the grism is composed of
a prism the vertex angle of which can be varied and a volume phase holographic
(VPH) grating. More specifically, the grism comprises a prism the vertex angle
of which can be varied; and a VPH grating being a diffraction grating and disposed
substantially orthogonal with respect to a side including the vertex angle of the
prism; the VPH grating being rotated around an axis being substantially orthogonal
to the side of the prism, whereby a slope angle defined by the VPH grating and
a predetermined plane is varied.
