What instrument is designed to measure the sagitta of a lens curve?

The lens clock is specifically designed to measure the sagitta, or the curvature, of a lens. It works by using a set of two opposing legs that are placed on either side of the lens surface. When the instrument makes contact with the lens, it measures the distance between the top of the lens and the flat plane created by the legs. This distance is the sagitta, which is crucial for understanding the curve of the lens and ensuring that it fits correctly into frames or works effectively for vision correction.

This instrument is particularly valuable in ophthalmic dispensing as it provides the curvature information needed to determine the lens's shape. Other options, such as a lensmeter, are used primarily to determine prescription strengths and focal points, while a polariscope is used for stress testing in lens materials. The ophthalmoscope, on the other hand, is an instrument for examining the interior structures of the eye, such as the retina, rather than measuring lens curvature. Therefore, the lens clock is the most appropriate choice for this specific task.