Newton ring experiment is a classical experiment about equal thickness interference．The curvature

radius of a plate-convex lens can be measured using the Newton ring method．One important step in this method

is to transform the formula into a measurable form． It is commonly considered that the reason for this

transformation is that one cannot know the center“or”the order．We will show that both the center“and”the

order cannot be measured．Meanwhile，we also prove that the data can be processed with linear fitting method．