132 



THE ART OF PROJECTING. 



prism is turned. If it be put close to the prism there 

 may appear a system of concentric circles about a uni- 

 form-colored field in the centre. The colors which 

 this central field assumes when the analyzer is rotated 

 are often superb. 



Spectacle glasses that are usually called Brazilian 

 pebble are made of quartz, and such will exhibit 

 brilliant colors by projection in plane polarized light. 

 This serves for a test of their genuineness, as glass 

 will give no such effect. ^ 



13. The system of bands and colored 

 curves seen in biaxial crystals is not 

 easy to project, because the angles at 

 which these are to be seen are so great. 

 With some crystals of potassium nitrate 

 it is possible to show both axes at a 

 time with the same arrangement as was 

 described for calc spar. A clear crys- 

 tal about the quarter of an inch in 

 diameter, and the twentieth of an inch 

 thick, may answer for this. Such small 

 crystals are usually mounted in a disk 

 of cork. 



Fig. 98 represents the double system 

 of rings and brushes seen in a crystal 

 of nitrate of potash, where the plane 

 of its axes coincides with the plane of 

 the polarizer ; and Fig. 99 shows the 

 appearance when the planes are slightly inclined to 

 each other. 



14. There are many minute crystallizations, such as 

 are prepared for the microscope, that make fine objects 

 when projected in polarized light. These objects may 

 be prepared beforehand ; or the crystallization with the 



mg. 98, 



Fig. 99. 



