CHEMICAL SELECTION OF MOTHERS. 121 



conditions the fatigue from this constant work is very 

 much lessened; many chemists use a large black 

 obstructor. The reading of the vernier depends upon 

 the light thrown upon it from an upper reflecting mir- 

 ror. It is most desirable that the chemist doing the 

 work be absolutely in the dark, and, as he is under 

 great pressure, he should be relieved every few hours. 

 Whenever the observer is changed the zero point of the 

 instrument should be verified. 



