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．