Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/248

 twenty others, of each of which I possess very numerous isolated crystals. Amongst these, the red oxide of copper already mentioned is one, though of comparatively little importance; because, as the real value of the angles of the cube, which it occasionally assumes, are known, the rest may be calculated with precision.

But there is still another circumstance which forcibly shews the great difficulties attending the accurate admeasurement of the angles formed by the meeting of the natural surfaces of crystals, by subjecting those surfaces to either goniometer. It is not often found that the best selected afford incidences corresponding with the results obtained by calculation, even where we must be assured that calculation may be relied on; as is the case in respect of all those substances which assume the cube, the regular octahedron, the rhomboidal dodecahedron, or the hexahedral prism. The angles formed by the meeting of the planes of these solids are known, yet rarely do the best selected crystals accurately afford the admeasurements which belong to them in their perfect state. It is not sufficient that they approximate; they should be precise.

The foregoing are my principal reasons for adopting the belief that many of the determinations of Haüy in regard to the admeasurements given by him of the primitive crystals comprehended in the term parallelepiped, and of the numerous varieties of the octahedron, will be found inaccurate. It is probable that the reflecting goniometer will render service to science by detecting some of the inaccuracies incident to the use of the common goniometer.

The accuracy of which the reflecting goniometer is capable, requires, for the reasons above assigned, great patience and perseverance in measuring the angles of crystals on their natural planes; and the chief, if not the only, difficulty attending its use,