442 Scientific Proceedings, Royal Dublin Society. 



The sensitiveness of the method is evidently controlled by the amount 

 of divergence given to the lines. In the case of objects of sharp definition 

 a small divergence may be used ; in the case of objects of inferior definition 

 it is better to use a larger angle of divergence. 



I venture to draw attention to the method, as I have nowhere seen it 

 described, obvious as are its principles. 



IvEAGH Geological Laboeatory, 

 TbinitxCollegb, Dublin. 



