Abstract: An apparatus for measuring the index of refraction of a specimen includes a stage plate having a small exposure opening behind which are located a point source infra-red light emitting diode and a photoresistor. A first lens focusses the LED point source along a first optical axis at the exposure opening on the plane of the specimen receiving face of the stage plate defining a sensing area and a second lens focusses the image on the sensing area along a second optical axis onto the photoresistor. The optical axes form equal angles to the perpendicular to the exposure plane at the center of the sensing area. The light emitting diode and the photoresistor are connected through a common normally open switch to a battery.