Combination of a reaction apparatus and an optical instrument for monitoring polymerase chain reaction replication of DNA in the reaction apparatus, the reaction apparatus including a thermal cycler block for holding a plurality of vials, each containing a suspension of ingredients for the reaction, the ingredients including a fluorescent primary dye that fluoresces proportionately in presence of DNA, the optical instrument comprising: a light source for emitting a source beam having at least a primary excitation frequency that causes the primary dye to fluoresce at an emission frequency; first means disposed to be receptive of the source beam to effect an excitation beam having the excitation frequency; primary focusing means (3, 2a, 2b) disposed to focus the excitation beam simultaneously into a plurality of suspensions such that the primary dye of each suspension emits an emission beam having the emission frequency, the emission beams having an intensity representative of concentration of DNA in each suspension, the primary focusing means (3, 2a, 2b) being receptive of and passing the emission beams; second means disposed to be receptive of the emission beams from the primary focusing means (3, 2a,2b) so as to further pass the emission beams at the emission frequency; emission focusing means (3, 2a, 2b, 9) for focusing the emission beams; a detector disposed to be receptive of the emission beams from the second means and the emission focusing means (3, 2a, 2b, 9) such that the emission beams are focused onto the detector, the detector generating primary data signals representative of the emission beams and thereby a corresponding concentration of DNA in each vial; one or more folding mirrors, and processing means receptive of the primary data signals for processing the primary data signals to compute the corresponding concentration of DNA; wherein the optical instrument is arranged without a beam splitter, with the first means constituting an excitation filter for the excitation frequency, with the second means constituting an emission filter for the emission frequency, with the excitation and emission beams being on slightly different optical paths angularly with the one or more folding mirrors provided such that the light source and the detector are not side by side. a light source for emitting a source beam having at least a primary excitation frequency that causes the primary dye to fluoresce at an emission frequency; first means disposed to be receptive of the source beam to effect an excitation beam having the excitation frequency; primary focusing means (3, 2a, 2b) disposed to focus the excitation beam simultaneously into a plurality of suspensions such that the primary dye of each suspension emits an emission beam having the emission frequency, the emission beams having an intensity representative of concentration of DNA in each suspension, the primary focusing means (3, 2a, 2b) being receptive of and passing the emission beams; second means disposed to be receptive of the emission beams from the primary focusing means (3, 2a,2b) so as to further pass the emission beams at the emission frequency; emission focusing means (3, 2a, 2b, 9) for focusing the emission beams; a detector disposed to be receptive of the emission beams from the second means and the emission focusing means (3, 2a, 2b, 9) such that the emission beams are focused onto the detector, the detector generating primary data signals representative of the emission beams and thereby a corresponding concentration of DNA in each vial; one or more folding mirrors, and processing means receptive of the primary data signals for processing the primary data signals to compute the corresponding concentration of DNA; wherein the optical instrument is arranged without a beam splitter, with the first means constituting an excitation filter for the excitation frequency, with the second means constituting an emission filter for the emission frequency, with the excitation and emission beams being on slightly different optical paths angularly with the one or more folding mirrors provided such that the light source and the detector are not side by side.