DALY. — ETCH-FIGURES ON AMPHIBOLES. 377 



those already established for the corresponding face and mineral group, or 

 with analogous figures belonging to other species. Furthermore, such a 

 description may be made in cases where any determination of the indices 

 of the figure-faces is impossible on account of the absence of " Licht- 

 schimmer," due to various causes, as fibrosity, minuteness of figures, cur- 

 vature of the surface studied, etc. But it is necessary to recognize that 

 a complete analysis of a figure is not j^ossible in many cases, even under 

 the most favorable circumstances. Tliis is true, for example, of the pits 

 on the prismatic faces of amphibole and in the vertical zone of most 

 monoclinic minerals. Relatively low powers of the microscope must 

 always be used, since contrasts of dark and light are speedily lost above 

 200 diameters, and thus it often cannot be decided whether an apparently 

 rounded figure-face may not really be one compounded of many small 

 faces, according to the well known examples of Becke, Baumhauer, and 

 others. Hence, inasmuch as it is not practicable to determine in the 

 microscope the elements (faces, angles, symmetry, etc.) with the same 

 precision and detail with which we can define a crystal, it becomes ad- 

 visable to choose certain elements of the figure that are sufficient to fix 

 its general shape. Such elements will be those which can be directly 

 measured in the microscope and with a maximum of exactness. They 

 will include straight sides and the angles between them as well as the 

 special angles between curved sides characteristic of each figure. These 

 elements, too, had best be such as can be recognized on very small figures 

 of a given category, since in some varieties it may be feasible to produce 

 figures only relatively very minute. Lastly, we must have a base-line of 

 reference for all measurements ; — in amphiboles, there is an excellent 

 one, the trace of the cleavage, which generally makes it unnecessary to 

 search out the directions of edges bounding the crystal-face. 



For the convenient examination of figures on (110), it is well to use 

 prisms with sides as smooth as possible, so that the mineral will lie flat, 

 and the plane to be studied perpendicular to the axis of the microscope. 

 In the study of terminal planes, or of material with which such perfect 

 prisms are not obtainable, the crystal or cleavage piece may be readily 

 brought into the desired position by mounting it on an object-glass with 

 wax and then adjusting it so that the simultaneous reflection of a ray of 

 light from the glass and the plane may occur. "With lustrous faces, this 

 adjustment can thus be carried out with a close degree of accuracy. 



The microscope used was a Nachet, provided with an apparatus for 

 vertical incidence of the light that illuminated the crystal-face. The 

 light was led through a collimator attached to an Auer lamp. Below 



