The meter exposes a small area of a strip of light-sensitive paper, which darkens on exposure. The time for the sensitive paper to darken to match the tint of the surround of the slit is the third input parameter to calculate appropriate exposure time for a given f-stop and plate speed, with help of the instrument's calculation scales.
It is a small tin which measures 1 1/2 inches in diameter. The Snipe meter was for use with simple cameras having fixed apertures and speeds. It does not give the exposure but indicates if the light is strong enough for an exposure. Made in England.