It consists of a very powerful incandescent lamp inclosed in a
metallic cylinder. One of the two semi-cylindrical sides constitutes
the reflector, and the other, which is of thick glass, allows of the
passage of the luminous rays, which thus illuminate with great
brilliancy the strata of earth traversed by the instrument. The base,
which is inclined at an angle of 45 deg., 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 earth. The lamp is so
mounted that its upwardly emitted rays are intercepted.
The whole apparatus is 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 an automatic and
portable battery.
[Illustration: THE TROUVE ERYGMASCOPE.]
This permits of lowering and raising the apparatus at will, without
derangement, and without its being necessary to interrupt the light
and the observation.
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