When using Newton's ring to test lens surface radius of curvature, the air gap in between the master plate and the lens can have either a positive or a negative meniscus shape. The optics for the negative meniscus shape air gap has been widely analyzed and the results are assumed to be valid for the positive meniscus shape air gap. Accurate and approximate analysis are performed in this paper for the positive meniscus shape air gap. Analysis results of this paper show that for the same parameters involved, the Newton's ring number for the positive meniscus shape air gap is larger than the Newton's ring number for the negative meniscus shape air gap. The difference between these two Newton's ring numbers increases as the value of h / r increases, where h is the lens half diameter and r is the master plate surface radius of curvature. For h / r = 0.87, the ratio of these two numbers can be as large as 2.
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