CH. XV] 



EXPERIMENTS WITH POLARIZED LIGHT 



623 



3. Pile of glass plates to polarize light. Prefer- 

 ably thin sheets of plate glass, but a pile of lantern- 

 slide cover-glasses can be made to answer. 



4. Plano-convex condensing lens, 5 to 10 cm. in 

 diameter and 20 to 30 cm. focus. This lens must be 

 free from strain. 



5. Projection objective, preferably of large diam- 

 eter and short focus. 



6. Two sets of lenses to give converging polarized 

 light. Two substage condensers of microscopes will 

 answer if free from strain. 



7. Objective of short focus and large diameter. 

 A plano-convex lens will answer (see 881). 



8. Specimens. Pieces of mica, crystal sections, 

 plate of glass on which crystals have formed, an- 

 nealed and unannealed pieces of glass, clamp for 

 putting the glass under strain when in the field of 

 the lantern. 



Many of the most beautiful experiments in optics 

 require the use of polarized light. The demonstra- 

 tion of this phenomenon is growing more difficult 

 owing to the increasing scarcity of the natural 

 mineral calcite, which is used to make the Nicol 

 prisms needed for polarizing and analyzing the 

 light. Clear pieces of calcite are getting so rare 

 that except for a few large Nicol prisms in private 



FIG. 350. 



PROJECTION WITH POLARIZED LIGHT, USING SMALL 

 NICOL PRISMS. 



L Source of light, right-angle arc. 



C t Ordinary magic lantern condenser. 



W Water-cell. 



NI Nicol prism of I to 2 cm. opening (polarizer). 



C 2 Condenser free from strain which renders the polarized 

 light parallel or slightly converging. 



5 Specimen. 



Magic lantern objective. 



N 2 Nicol prism (analyzer) of i to 2 cm. opening. 



