Made by Atago Optical Works, Japan, this device is to measure sugar level in solutions. The refractive index of a solution is proportional to its sugar concentration. As could be seen in below diagrams, as the light enters its optical barrel, it goes through its front prism, and through the solution applied between portions I, and H. This is very similar to an Abbe Refractometer.

There is a rotary Amici prism G that eliminates the color, to provide a high contrast between the dark, and light fields of the image viewed through the eyepiece. The image would look like a circle whose bottom half is dark, and top half is bright. By turning the dial L, an scale is read where the boundary line of critical ray falls on. The readout from a thermometer is added or subtracted from this reading to get the percentage of sugar level. For example if a read value is 23.7%, and the thermometer indicates at +0.2, the value is adjusted as 23.7+0.2=23.9%

Atago Hand Held Refractometer

Atago Hand Held Refractometer model 500

Specimen Lid,

Bottom Half Prism I

Eyepiece A

G

H

L