An optical instrument (A) for monitoring polymerase chain reaction replication of DNA may include a thermal cycler block (1c) for holding a plurality of vials (1b), each containing a suspension of ingredients that include a fluorescent primary dye. The instrument may include a light source (11) for emitting a source beam, a first means (7) disposed to be receptive of the source beam, a primary focusing means (3) disposed to focus the excitation beam simultaneously into a plurality of suspensions and being receptive of and passing emission beams, a second means (8) disposed to be receptive of emission beams, an emission focusing means (10) for focusing emission beams, a detector, and a processing means for computing concentration of DNA. The optical system being arranged without a beam splitter (6) but with one or more folding mirrors (5), and with excitation and emission beams being on slightly different optical paths angularly.