Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/357

Rh instructions from the ingenious and scientific inventor. Before I had arrived at some tolerable knowledge in its use, so as to be assured that the smaller crystals only can be relied on, the great differences which I found to exist in the same angles of the larger crystals, even though their planes appeared by the assistance of the magnifying glass, to be undeviating and polished surfaces, almost tempted me to doubt the utility of the instrument itself. These differences amounted in many instances to as much as 15', frequently 10′; while on the other hand, small crystals, having clear and perfect reflections, gave a coincidence in the admeasurement of the same angle. The reflecting goniometer is so delicate an instrument, that great care is requisite in the choice of the crystals subjected to it for the admeasurement of their angles. It often happens that those of apparently the most beautiful surfaces are unfit for this purpose; the most clear reflections alone can be relied on, and even then only by comparing the results of trials on many crystals. Some of the first attempts gave an incidence of 2 on 2 over the apex of the fig. 27. Pl. 16. one way of 92°.55′. the other way 93°.20′. or even 93°.25′. and this induced the suspicion that the bases of the two pyramids composing the primitive octahedron, were not square. The crystals on which those admeasurements were taken, were, comparatively, large, and their reductions were by no means so clear as those since obtained on much smaller ones, which have confirmed the real incidence both ways to be 92°.55′ and therefore that the common base of the two pyramids is square.

The crystals of this substance are likewise subject to another difficulty, that of a double reflection, even on faces which, by the assistance of the lens, appear of the most perfect kind. I possess a crystal giving two reflections on three of the four faces, 2, 2 fig. 27. which are those of the pyramid commonly observed on the crystals of this substance. The incidence obtained one way over the apex, with the two strongest reflections was, 92°.55′. with the two weaker 93°.10′. but with a strong reflection out one face and a weaker on the other 93°.5′. On one of the other two opposed faces of the pyramid, one reflection only was given, but on the other, two were visible; with the strongest reflection, the incidence obtained was 93°.S5′. with the other, 93°. 25′. the least of them 30′ above the real value of the angle.

I feel therefore warranted in the conclusion that, although

Rh