£52 Goldschmidt and Nicol — New Forms of Sperrylite. 



rag them by lithographic plate, only part of the crystal is shown. 

 The forms x and h are wanting, as their existence was decided 

 with certainty by measurements made after the drawing was 

 prepared. In tigs. 2-5 the attempt has been made to preserve 

 the proportions of the faces as they actually occur in nature. 



The measurement, or rather the discussion of the results 

 of the measurement^ offered several difficulties which can be 

 referred to the small size of the crystals and especially of the 

 subordinate faces. These faces are not only absolutely but also 

 relatively small, that is, when compared as to size, with the 

 principal faces, c and p, they stand far in the background. 

 This may be readily seen from an inspection of the figures. 

 In discussing the measurements in the gnomonic projection, 



the following unexpected phenomenon was observed. In cases 

 where the principal faces c and p occupied their exact position, 

 showing that the crystal had not been disturbed in its growth, 

 yet the signal reflections from the subordinate faces were dis- 

 placed in many ways, and this, too, when these faces were well 

 defined as to lustre and perimeter. The minuteness of the 

 subordinate faces is exceptional. It is interesting to note what 

 minute faces can be measured by the excellent instruments of 

 the present day. The following measurements were obtained 

 under the microscope : 

 Crystal No. 4, Fig. 3. 



The large upper c face, longest diameter — 0-8 nim 



The large front p face, " " = 0-6 



The upper e face, to the right from p, " = 0*06 



The lower if/ face, to the right from p, " " = 03 



The lower x face, to the left from p, " " = 0'03 



The lower 6 face, length = 0'3 mm ; width — 0*015 



The displacements and consequent variations in the case of 

 such small faces may have two causes : 



1. Optical displacement and diffusion of the signal reflection 

 owing to bending of the light. The smaller the face the stronger 

 is this effect and the less the certainty in determining the posi- 

 tion of the face. 



