Sentence:  Essentially these <WSD>birefringence</WSD> studies show that at low rates of shear a tension is present at 45 ` to the direction of shear , and as the rate of shear increases , the direction of the maximum tension moves asymptotically toward the direction of shear . 
Meaning of the word marked with <WSD> tags:  splitting a ray into two parallel rays polarized perpendicularly