Abstract: The relation between the height of the measured object and measuring precision is analyzed in the experiment measuring the index of refraction with macroscopic method;and it is also demonstrated how to select the height of the measured object to ensure clear image and least measuring error.It is concluded that the height of the object Ｈ should satisfy:Ｈ≤ｎｈ；on the other hand,the closer　Ｈ　is to ｎｈ,the higher the measuring precision is.In a word,the height of the object should be slightly smaller than　ｎｈ.

Key words: height, maximum value, index of refraction, data, error