108 A HALF-HOUR WITH 



undulations of white light thus , f , and ; 

 and if these are placed so that their positive 

 axes (marked P A) coincide, they give the sum of 



Fig. 28. 



their combined retardations. If any be turned 

 until its P A is at 90 to the P A of the others, 

 the lesser number is subtracted from the greater. 

 For instance, when the P A of the f is placed at 

 right angles to the P A of the the sum of the 

 difference is obtained =%; if the is now added 

 with its P A coinciding with the P A of the , % 

 are obtained ; but if placed to coincide with the 

 P A of the f , -f- is the result. 



Therefore by subtracting by 90, or adding by 

 the P A, any number from \ to J T 3 , undulations 

 may be retarded which includes all the colours of 

 the spectrum. 



To those who may wish to try the effect of 

 polarized light at a small cost, the following plan, 

 suggested by Professor Reinicke* will be found 

 useful. 



Procure from twenty to twenty-five pieces of 

 thin covering glass flat and free from veins. The 

 size most convenient for the purpose is 18x12 

 mm. Fig. 29 represents the exact size. These are 

 to be fixed on a tube at an angle to the tube of 



* The Professor says 50 to 60, but with that number the 

 loss of light is considerable. 



