Abstract: This invention relates to an optical system which will enable easy use in the field. Known systems either require sample and reference targets to be imaged within the same telescope field or do not enable accurate identification of the measuring field in a view finder.The present invention overcomes this problem by providing a system having a first branch (10, 12) and a second branch (14) which receive light from a sample and reference target respectively. A shutter 22 sequentially allows light from the two branches to pass to a mirror 20 having a slit 30, reflective portions 32 and transparent portions 34. Light from the first branch passes through portions 30 and 34 to a detecting branch (42, 46, 50-56) and some light is reflected through a lens 16 to eye piece 19. Similarly some light from the second branch passes through mirror 30 and 34 to eye piece 19 and some light is reflected to the detecting branch.