enter a first rotary sector 3 where the beam is alternately divided or distributed between two beam paths the first sector 3 has reflective and transparent regions arranged in a particular pattern as will be described later the light beam reflected by the first sector 3 is reflected by plane mirrors 4 and 5a to travel along a reference beam path 14 to form a reference beam whereas the transmitted light beam from the first sector is reflected by a plane mirror 5b to travel along a sample beam path 15 to form a sample beam the beams travelling along the reference and sample beam paths 14 and 15 enter reference and sample cells 16 and 17 supported in a cell holder 6 and aligned with the reference and sample beam paths respectively and are then reflected by plane mirrors 7 and 8 a second sector 9 is positioned substantially at the crossing of the thus deflected beam paths the reference and sample beams enter the second sector 9 where they are guided into a common beam path more specifically the second sector 9 also has reflective and transparent regions arranged in a particular pattern as will be described later in detail the reference beam passing through the second sector 9 and the sample beam reflected by the second sector 9 alternately appear in the common beam path the light beam from the second sector 9 is focused to an incident slit of a monochromator 12 through a concave mirror 10 and a plane mirror 11 the light beam is spectrally isolated by the monochromator 12 into monochromatic light which in turn enters a detector 13 in the form.