minerals, having the form of peridot in the slags of the furnaces of Sweden and Germany, (Brewster's Journal, Vol. 1. p. 130, and Annales de Chemie et de Physique, Tom. XXIV.) Some of the varieties, of which he gives the analysis, consist almost entirely of protoxide of iron and silica, in the proportion of 36 of the former, to 16 of the latter; in others he found lime or magnesia, replacing part of the iron, so as always to make up a quantity of oxygen equal to that of the silica. These chemical constitutions may be expressed by the formulæ and all these cases are included in the chemical formula Fe M+S where Fe, M, C, &c. are isomorphous or plesiomorphous; that is, may replace each other with no change of form and angle, or with a slight change of angle, and a preservation of the system of crystallization. The mineral called Lievrite or Yenite has for its formula 4 (Fe + S) + (C + S). This is manifestly a particular case of the formula already given. Also Zc + S, the formula expressing the composition of silicate of zinc, is analogous to it, Zc being isomorphous with Fe. We might expect therefore the angles in these two cases to approximate to those of peridot. It will however be more convenient to compare them with the angles of the slag, than with those of peridot, the former having a greater number of secondary planes. In cases where any of the planes are wanting, it will be necessary to complete the series, and to compute the angles between the interpolated planes. The results of the comparison are presented in the following Table. XX. On the Improvement of the Microscope. By H. CODDINGTON, M.A. F.R.S. FELLOW OF TRINITY COLLEGE, AND OF THE CAMBRIDGE PHILOSOPHICAL SOCIETY. [Read March 22, 1830.] AMONG the numerous excellent suggestions which Dr. Brewster has from time to time thrown out to those engaged in the theory or the practice of Optics, there is one which appears to have been most unworthily, and most unaccountably, neglected. It is that of substituting a sphere for a lens in the construction of a microscope. This is the more surprising, as many persons of great eminence have, of late years, turned their attention to the improvement of this instrument, in which pursuit they have spared neither time, labor, nor expense. The only reason which I can give for this is, that as, until the investigations of Professor Airy, which are contained in the present volume of the Transactions of this Society, nobody, with the exceptions of Dr. Young and Dr. Wollaston, ever dared to approach the thorny subject of the oblique refraction* of a pencil of rays by a lens, almost * I allude here to the refraction of light, considered as homogeneous. The chromatic dispersion introduces another source of error, which has been very successfully overcome. I have found the colour most completely corrected in microscopes which were, in other respects, so ill constructed, as to be nearly useless. |