在牛顿环实验中，圆纹直径是有关测量的基础数据.为了提高测量的准确性，本文从理论上分析了引起系统误差的原因：一是直径没有经过圆纹圆心（引入不同的弦心距），二是直径（实际上是弦）的两个端点偏离光强极值位置.于是提出一种可以完全消除牛顿环测量系统误差新方法（共同弦线法）：以弦心距相等的各个圆纹的弦长代替直径，结合光强极值定位弦的两个端点.基于这种方法，对于牛顿环实验，提出两种测量新安排：对于使用读数显微镜情况，可以在牛顿环仪中对所有圆纹添加一条共同弦线，作为测量各个圆纹弦长的参考线，并用弦长代替直径，从而消除第一种系统误差；对于使用图像法情况，可以进一步获得共同弦线上的光强分布，通过光强极值定位弦的两个端点，获得圆纹上的弦的精确长度，从而消除两种系统误差.在新设计的牛顿环装置上应用图像法进行测量，证明此方法是有效和可行的.

In the Newton's ring experiment, the diameter of the circular fringe is the fundamental data of measurement. In order to improve the accuracy of measurement, this article theoretically analyzes the reasons that cause systematic errors: firstly, the diameter does not pass through the center of the circular fringe (introducing different chord-center distances), and secondly, the two endpoints of the chord deviate from the position of the extreme light intensity. So in theory, a new method is proposed to completely eliminate the above errors in Newtons ring experiment: replacing the diameters with the chord length of all circular fringes with equal chord-center distance, and combining the extreme value of light intensity to locate the two endpoints of the chord. Based on this method, two new measurement arrangements are proposed for the Newtons ring experiment: in the case of using a reading microscope, a common chord for all concentric circular fringes can be added to the Newtons ring instrument as a reference line for measuring the chord length of each circular fringe, and the chord length can be used instead of the diameter to eliminate the first systematic error; in the case of image analysis method, the light intensity distribution on the common chord can be further obtained, and by locating the two endpoints of the chord through the extreme light intensity, the accurate length of the chord of all circular fringes can be obtained, thereby eliminating the above two types of systematic errors. Then, the image analysis method is applied to the newly designed Newtons ring device for measurement, proving that this method is effective and feasible.