Joly — An Improved Polarizing Vertical Illuminator. 5 



being so placed that one part of it extends over the aperture. In 

 making this comparison it is well to stop off sufficient of the trans- 

 mitted substage light to make the twice transmitted and singly 

 transmitted rays about equally bright. The result is a valuable 

 discriminative test, as will be realized more especially by any one 

 acquainted with the work of MM. Levy and Lacroix, " Les 

 Mineraux des Roches," to which I have more fully referred in my 

 former paper. 



It will be seen from the above that in order to use the method of 

 examination by double transmission, the reflecting plate of specu- 

 lum metal (a small disc about 3 cms. in diameter is a convenient 

 size) and the vertical illuminator as described are alone required. 



It is worthy of notice that this mode of obtaining the effects of 

 a doubled thickness of the section by no means involves a doubled 

 opacity ; for inclusions, flaws, &c, lie above their own images, and 

 fresh ones are not added, # as would be the case in a section of 

 actually double the thickness. 



SCIEN. PROC. R.D.S., VOL. X., PART I. 



