For the given Rx, which number would be observed on the sphere power wheel of a manual lensometer for the flat-top segment?

In a manual lensometer, when measuring the prescription for a flat-top segment, the sphere power observed on the sphere power wheel corresponds to the spherical correction that applies to the flat-top segment of the bifocal or trifocal lens.

The flat-top segment is designed primarily for reading, which typically requires less minus power compared to the distance portion of the lens. In this case, a spherical power of -1.25 suggests that the segment is providing a moderate correction for near vision. This is a common value for many patients who have presbyopia, as they may require less correction for near tasks as they age.

The other values provided, being more negative, represent stronger corrections that are typically needed for distance vision considerations rather than near vision. Therefore, the presence of -1.25 as the observed power on the sphere power wheel accurately reflects the requirement for a flat-top segment, which is intended for close-up work. Understanding how these values relate to different tasks—such as reading—helps clarify why -1.25 is the correct observation for the flat-top segment.