225 



On a New Analyzing Selenite Stage. 



By W. Hislop, F.R.A.S. 



(Read August 27th, 18G9J 



The method of illuminating objects by means of a polarized beam 

 of light has long been before microscopists, but little has hitherto 

 been done to make this system of microscopic examination available 

 for scientific investigation, or in other words, for real work. It has 

 indeed been very successfully used for bringing out the beautiful 

 play of light, shade, and colour, exhibited by many objects having a 

 doubly refracting structure, and many contrivances have been 

 adopted for making its application for these purposes more 

 complete and perfect. Beyond all this, however, there is a value 

 to be attached to its use which has been, as yet, appreciated by 

 comparatively few microscopists. This may be due partly to the man- 

 ner in which the apparatus is generally arranged, giving little facility 

 of adjustment to suit different conditions, and consequently often pro- 

 ducing the impression of uncertainty on the mind of the manipula- 

 tor, as to the result to be obtained. 



Fig. 1. 



A 



+ 



In order to point out how this uncertainty may arise, I must, 

 for a moment, recall the conditions of polarization. Let A, Fig. 1, 

 represent an ordinary beam of light, either daylight or emanating 

 from a lamp. B is the polarizer. This may be a piece of glass 

 blackened at the back, and used as a reflector, or it may be a 

 bundle of thin glass plates similarly used, or placed at an angle 

 with the axis of the beam of light, which is then partly transmitted 

 and partly absorbed. It may be a thin plate of tourmaline or a 



