Erygmascope

One of the two semi-cylindrical sides constitutes the reflector, and the other, which is of thick glass, allows the passage of light, which illuminates the strata of earth traversed by the instrument.

The base, which is inclined at an angle of 45°, is an elliptical mirror, and the top, of straight section, is open in order to permit the observer standing at the mouth of the well, and provided with a powerful spyglass, to see in the mirror the image of the geological layers or of the structure of crystalline rocks.

The whole apparatus was suspended from a long cable, formed of two conducting wires, which winds around a windlass with metallic journals which are electrically insulated.

These journals communicate, through the intermedium of two friction springs, with the conductors on the one hand and, on the other, with the poles of a portable battery.

The etymology of the term remains enigmatic, but could originate from a Greek word ήρυγμα meaning "trench".