This is a wonderfully simple piece of darkroom equipment. Basicly it is a magnifying glass and mirror. It provides the user with the optimum sharpness for the enlargement by focusing on the grain of the negative.
Also known as a grain magnifier it rests on the masking frame where it diverts a small amount of light from the projected negative to your eye as you look through the small magnifying lens. With one hand on the focusing control of the enlarger you gently turn the knob until the grain becomes sharp in the mirror. This translates to a sharp image at easel level; to maintain this accuracy a piece of waste photographic paper should be placed in the masking frame with the magnifier on top. It is easiest to focus on the grain with the enlarging lens set at its widest aperture and then re-checked at the working aperture. To make sure that the focus has not changed.
Some focus finders can only be used in the central area of the projected image. This is because the angle of the light is more severe at the edges and corners. If you wish to check these areas to make sure that the whole negative is in focus you will need a magnifier that allows for this with a wider mirror and tilting aim.