100 



of structure alone, it may be considered a boon. Bio- 

 logical work cannot be successfully prosecuted with- 

 out it. 



The polariscope is absolutety requisite in petro- 

 graphical work, and in the examination of crystals in 

 connection with a selenite, it is an excellent appar- 



Figs. 21. 



atus in the hands of professional and amateur for 

 instruction and amusement, bringing out wonderful 

 coloration of many objects. Although the microscope 

 is really an instrument for scientific research, it does 

 not suffer by being used as a means for recreation 

 and pleasure, and in this direction no accessory will 

 aid more than a polariscope. Three selenites may be 

 used with it, giving the following combination of col- 

 ors : Blue and yellow, red and green, purple and 

 green. The polarizer is always arranged to resolve. 

 The bull's-eye condenser is mainly intended for the 

 illumination of opaque objects, and although modern 

 instruments have their mirrors so hung that they may 



