Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/249

 is the finding of crystals that are adapted to it. It is requisite not only that their planes should be brilliant, but also that the image of the line or bar reflected upon them should be perfectly defined; its, edges should be seen with an exactness which they who have not given attention to the subject will scarcely believe to exist on surfaces so small. No reliance can be placed upon a solitary admeasurement taken upon the natural planes of one crystal, however brilliant and perfect they may appear; it is essential that several should be found to exhibit a perfect agreement, before it can be assumed that the actual value of any angle has been obtained. From one crystal of quartz, nearly perfect at both terminations and affording pretty clear reductions, I obtained six admeasurements of the same angle from its various planes. One of 94° 9′, another of 94° 12′, a third of 94° 16′, and three of 94° 11′. I did not discover one crystal whose planes were sufficiently perfect to enable me to obtain corresponding measurements from plane to plane, around it.

It cannot however, be doubted, that much of the labour attendant on the use of the reflecting goniometer in the measuring of the angles of crystals on their natural planes, will be spared, and that the results will be much more satisfactory, because more accurate, whenever cleavages can be obtained sufficiently brilliant for its use. In almost every instance in which I have tried it upon the planes produced by good fractures, the success has been complete. The great value of this instrument, which may be used with readiness and case, demands the attention of every one who has the slightest pretension to crystallographical research. I feel much pleasure in adding an instance of the accuracy of which it is susceptible. Having seen in some periodical publication, a notice that Dr. Wollaston had discovered differences in the value of the angles of the primitive