THE 



SCIENTIFIC PROCEEDINGS 



OF 



THE EOYAL DUBLIN SOCIETY. 



AN IMPROVED POLARIZING VERTICAL ILLUMINATOR. 



By J. JOLY, M.A.,D.Sc.,F.R.S., Professor of Geology and Mineralogy, 

 Trinity College, Dublin; Hon. Sec. Royal Dublin Society. 



[Read December 16, 1902 ; Eeceived for Publication January 1 ; 

 Published March 9, 1903]. 



In the Scientific Proceedings of this Society, vol. i. (n. s.J, p. 485, 

 et seq., I have described a method of observing on an ordinary 

 rock-section the interference tints proper to double the thick- 

 ness of the section, and thereby producing discriminative effects 

 not possible to obtain in the ordinary mode of observation. 



I may recall that the method consists in placing a plane 

 reflecting surface (polished speculum metal, preferably) beneath 

 the rock-section as it rests on the stage of the microscope, and 

 transmitting, by means of any vertical illuminator (as used for 

 examination of metals, &c), a plane polarized ray vertically 

 downwards through the rock-section. The ray reflected from the 

 speculum metal is again returned through the object-glass, and, 

 after passing through the analyser, shows to the eye the retarda- 

 tion proper to double the thickness of section. In this manner 

 the range of colour-variation from one species to another is 

 greatly increased : in fact, what differences exist for the single 

 thickness are now doubled in amount. 



In the different arrangements proposed to effect this, one objec- 

 tion, in some degree, applied in all cases : a want of verticality in 



SCIENT. PROC. R.D.S., VOL. X., PART I. B 



