S88 C, Palache — Crystal Measurement, etc, 



side at the edge of H is fixed a vertical scale M, which receives 

 projections as of prisms and steep pyramids which would fall 

 outside the paper. By means of a ruler at the foot of M a 

 mark is made on the paper showing the direction of such pro- 

 jections (the meridian). 



The use of the instrument is ordered as follows : Paper 

 being mounted on the horizontal circle and the crystal attached 

 to the carriage in as near the desired position as possible, the 

 carriage is brought upon H and the crystal brought to the cen- 

 ter of the instrument. The adjustment of the crystal in polar 

 position is similar to the method explained for the other instru- 

 ments. If the polar face is present, the vertical circle is set at 

 0° and the crystal tipped by means of the adjusting key until 

 the image reflected from the face is at the crosshairs of the 

 telescope and remains stationary during a revolution of H. If 

 the adjustment is to be made by means of the prism zone, Y is 

 clamped at 90° and the adjustment made for two adjacent faces 

 of the zone successively. 



The crystal being oriented, Y is brought to 0°, the crystal 

 removed on its carriage and the needle is pressed down to 

 mark the projection of the pole on the paper. The needle 

 being withdrawn and the carriage .replaped, a new face is 

 brought to reflection by movements of H and Y, both are 

 clamped, the crystal removed as before, the point of projection 

 pierced on the paper and readings taken on H and Y for (f> 

 and p. This process is repeated for each face, is rapid and 

 involves no special difiiculties. 



All the faces having been projected, the paper is removed 

 from the instrument and mounted on a large sheet for the pur- 

 pose of plotting the points which fell without its surface. The 

 resulting projection is discussed in the same manner as the 

 ordinary gnomonic projection. For further details concerning 

 the use of this novel instrument, the discussion of the result- 

 ing projection, and for examples of measurements carried out 

 upon it showing the remarkable accuracy of the results ob- 

 tained, the reader is referred to the paper by Prof. Goldschmidt. 

 Ein Projectionsgoniometer, Zeit. fiir Kryst., xxv, 1896, p. 538. 



It is beyond the scope of this paper to describe the system 

 of form development based on " force symbols," with its new 

 elements and symbols, which has been developed by Prof. 

 Goldschmidt. With the Index der Krystallformen, the forth- 

 coming table of angular coordinates (Winkeltabelle) for all 

 minerals, and the goniometer with two circles which is its 

 logical outgrowth, it forms a symmetrical system destined to 

 vastly simplify the work of practical crystallography and to 

 increase its scope and accuracy. 



Mineralogical Laboratory, Harvard University, May, 1896. 



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