NA VIGA TION. 



division are read off on the scale of the vernier, when 

 they cannot be estimated directly with sufficient 

 accuracy and readiness. A small magnifying lens is 

 generally used in reading the scale with or without 

 the vernier. 



8. Take now the instrument in your hand, and 

 look at a distant object, A, through the unsilvered 

 plate-glass, and turn the silvered mirror till the 

 ghost of the same object A, seen by the double 

 reflection, coincides precisely with the object itself 

 seen directly. Then read, on the graduated scale, 

 the number corresponding to the position of the 

 marker carried by the turning arm. Suppose the 

 reading to be 5'^. This is what is called the index 

 error of the instrument. Now take the instrument 

 in your hand again, and turn the arm carrying the 

 silvered mirror round till the ghost of one object, B, 

 seems coincident with the substance of another, 

 A, seen through the unsilvered glass. Look at the 

 scale again and take the reading, say 117 8': 

 subtract the index error from this and you find 

 1 17 2'f, which is the angle between A and B as 

 seen from your actual position. 



