ATC optical salinity refractometer

A handheld optical ATC refractometer for reading the salt content of water -- the instrument for a marine aquarium, where salinity has to be held inside narrow limits. One drop is enough: place it on the prism, close the cover plate and read the scale through the eyepiece against the light. The scale is dual, 0 to 100 ppt of salinity and 1.000 to 1.070 specific gravity (d 20/20 at 20 C), so it reads in the same units every aquarium guide is written in. It works by refraction: the more salt a solution carries, the more it bends the light, and the boundary between the light and blue fields moves up the scale. ATC stands for automatic temperature compensation, which the instrument applies to the sample itself. Calibrate it to zero on distilled water before the first reading, with the screw under the cover; the screwdriver for it is in the box.