GONIO'METER is an instrument em ployed to determine the angle at which the planes of crystals are inclined to each other. The common goniometer consists of two arms of metal, like the letter X, turning on a joint at the intersection. Two contiguous planes of the crystal are placed between the shorter arms, and a graduated arc between the longer arms serves to measure the inclination of the planes. As this instrument is incapable of affording results sufficiently accurate to deter. mine the species to which a crystal belongs, we proceed to describe the more perfect in strument invented by Dr. Wollaston, and called the reflective gonionzeter. This instru ment consists of a graduated circle mounted upon a horizontal axis, to one extremity of which is attached a moveable pin, having a slit for the purpose of receiving a small brass plate. To this plate the crystal is attached by means of a piece of wax, so that it may pro ject beyond the edge of the plate. The pin (which is provided with a vertical and hori zontal movement) is then raised or lowered until the reflection of any convenient object above appears to coincide with some other ob ject beneath. The instrument being thus
adjusted, the graduated circle is turned until a similar reflection is obtained from the con tiguous side of the crystal. The arc which the circle will then have described will be equal to the supplement of the inclination of the crystalline planes ; but, the margin of the circle being graduated in an inverted order, the true inclination is given without further computation, and may be read off with consi derable accuracy. In turning the crystal, the direction of the edge common to its two planes should not be altered, and the rays iu both instances should be reflected from that portion of the planes which is nearest to their common edge, otherwise the observationwill be affected by parallax. By this instrument the inclina tion of planes, whose area is less than the 100,000th part of a square inch, may be de termined, by means of a vernier, within a minute of a degree.