Joly — The Penological Examination of Paving- Sets. 83 



APPENDIX. 



Estimating the Hakd and Soft Constituents. 



The problem of determining the proportions of hard and soft 

 constituents in the rock under examination is very simply and 

 readily solved in the following manner : — 



The thin rock-section is placed in the microscope ; and using a 

 low power (1 inch or lower) and low eye-piece, the image of the 

 field is projected on to a ground-glass screen above the eye-piece, 

 any of the usual photographic apparatus being used. The ground- 

 glass is turned rough side up. Upon this is placed a transparent 

 divided scale prepared as follows : — A piece of logarithmic paper 

 (divided to square millimetres or square tenths of inches) is placed 

 in contact with a sensitive plate in a photographic printing-frame, 

 and printed off by contact in the usual manner. The result is a 

 negative, having the divisions appearing as clear lines on a dark 

 background. This negative may be used if not too densely 

 developed, or from this a positive is printed. 



The transparent divided scale is placed face-downwards upon 

 the ground glass. We evidently now have an image of the field 

 traversed by the lines upon the scale. On the back of this scale 

 the outline of any particular constituent is traced by an ordinary 

 writing-pen and ink. This done, the divided plate is lifted off ; 

 and, holding it up to the light, the number of square milli- 

 metres or square centimetres are estimated as contained within 

 the ink outlines. The whole circular area of the field in square 



7T.D 2 



cms. is —j— ; and hence the area occupied by the mineral can be 



estimated as a percentage of the area of the field. This is done 

 for several fields, and an average taken ; thus, in the case of the 

 Arklow set described above, the approximate areas in square 

 centimetres occupied by the coloured constituents and ores in 

 successively taken fields were 14J, 27, 17|, 23, 17^, 13J, 16. The 

 mean is 18|. The field is just 10*1 cms. in diameter, or has an area 

 of 60 square cms. Thus the constituents other than felspars 

 amount to 23 per cent. ; or we may assume that the felspar 



