TRANSACTIONS OF SECTION C, 641 



The successes achieved in the artificial formation of minerals, the advances 

 made in the methods of discrimination of minerals by the blowpipe and micro- 

 chemical reactions, the increase in our knowledge of the modes of alteration of 

 minerals, of their association, of their modes of occurrence, must all be left 

 undiscussed. 



Instruments. — I may add a word relative to the instrumental appliances which 

 have been placed at the service of the mineralogist since the issue of Whewell's 

 Report. As regards goniometers, the provision of two mechanical circular move- 

 ments in perpendicular planes for the easier adjustment of a crystal-edge parallel 

 to the axis of the instrument, first suggested by Viktor von Lang when assistant 

 at the British Museum, has proved a great convenience and is njw in general use. 

 The employment of a collimator with interchangeable signals, of a telescope with 

 interchangeable eyepieces, and the provision of lenses and diaphragms for obtaining 

 images from faces so small as to be invisible to the unassisted eye, would seem to 

 have brought the reHective goniometer, the invention of our distinguished country- 

 man Dr. Wollaston, to a degree of perfection where further improvement is scarcelv 

 to be looked for; though two crystallographers, Fedorow and Goldschmidt, have 

 recently constructed instruments with an additional telescope and entirely different 

 arrangements. It may be worthy of remark that, though reflective goniometers are 

 generally made for use with very small specimens, one was constructed for the British 

 Museum some years ago by which it is possible to measure the angles of a valuable 

 crystal without removal of the specimen from a matrix of several pounds' weight. 



The polariscope for use with convergent light, the stauroscope, the employment 

 of polarised light with the microscope, the adaptation of the microscope Cor the 

 observation of the interference-figures yielded by extremely minute crystals, the 

 spectroscope in the investigation of selective absorption, have all proved of great 

 service in the advancement of our knowledge of the characters of minerals. 



Worthy of special mention is that recent addition to our resources, the total 

 reflectometer, an instrument by which it is possible to determine with wonderful 

 accuracy the refractive index or indices from observation of the reflected light. 

 The process was long ago suggested by Wollaston; but it is only within the last 

 few years that forms of instrument have been devised by Kohlrausch, Soret, Lie- 

 bisch, Pulfrich, and Abbe, which make the method as precise in its results as that 

 which depends on refraction by a prism. In its more refined forms the total 

 reflectometer has been used to test the accuracy of the form of Fresnel's wave- 

 .surface : in the convenient, though less precise, form devised by Bertrand, the 

 instrument is useful in the discrimination of the species of minerals. 



For the measurement of the optic axal angle, when the angle is so large that 

 the rays corresponding to the optic axes are totally reflected at the surface^of the 

 plate and do not emerge into air from the crystal. Professor W. G. Adams 

 made the valuable suggestion that the crystal-plate should be interposed between 

 two hemispheres of glass ; several instruments on this principle have been con- 

 structed abroad, and have only been imperfectly satisfactory, but one lately made 

 in this country for the British Museum, under the superintendence of my excellent 

 colleague Mr. Miers, proves to be most eflicient for the intended purpose. Mr. 

 Tutton's apparatus for supplying monochromatic light of any desired wave-length 

 is a noteworthy addition to the instrumental resources of the mineralogist. The 

 meldometer of Joly for the more accurate determination of the fusing point of 

 minerals should also be recalled to you. 



In this slight sketch it has been possible to make only the barest mention of 

 some of the more important results which have been arrived at since the issue of 

 VVhewell's Report. You will doubtless think that it must have been possible in 

 the year 1832 to look forward enthusiastically to the progress which was about to 

 be made. But though Professor Whewell was himself confident that valuable 

 discoveries would reward the mineralogical worker, he was sadly depressed, and, 

 I think I may venture to say, with good reason, by the neglect of mineralogical 

 study in this country. Ilis own words are : ' This decided check in the progress of 

 the science has, I think, without question, very much damped the interest with which 



