Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/259

 brilliant for the use of the reflecting goniometer; that instrument therefore, in regard to this substance, has been used only to measure the angles by means of the reductions of the natural planes of the crystals; but as the hyacinth of France is always too much water-worn to present those well defined reflections which alone can be relied on, and which frequently occur on the smallest and most transparent crystals of the jargoon, I first depended on the latter, but have since been enabled by the examination of a large quantity of hyacinths, to find some crystals which, though dull, afford the same results.

These results differ from those obtained by Haüy, no less than one degree and a half, which caused me to measure over again the whole number of crystals, but without discovering any error. The incidence of P on P is given both in the Traité and Tableau Comparatif as 82° 50′, leaving of course the incidence, of P on the opposed plane over the summit 97° 10′, as the complement. But as the crystals of jargoon in my possession, rarely exhibit both pyramids, and never sufficiently brilliant to be relied on, I have been compelled to depend on measurements obtained on the plane P and the opposed plane over the apex. Clear reductions agree in five instances in affording 95° 40′, in two or three 95° 55′, and in one instance 95° 30′; while the only incidence of P on P is 84° 15′, being five minutes short of what I conceive to be the true value of the angle, viz. 84° 20′. Two fragments exhibiting planes parallel with the faces of the primitive octahedron, but not sufficiently bright for the use of the reflecting goniometer, afford by that in common use, an angle of about 95° 40′; two others of about 84° 20′.

I have now stated the reasons which induce me to assume the true measurement of P on P to be one degree and a half greater