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, the measurement observed on the sphere power wheel for a flat-top segment typically corresponds to the power of the reading segment, which is usually less than the distance prescription and is used for near vision tasks. In this scenario, the correct answer indicates a power of -1.25 diopters.

This measurement is significant because flat-top bifocal lenses have a specific segment designed for reading or close-up work, which is different from the main lens's spherical power for distance vision. The segment power is generally lower in magnitude than the distance portion due to the different visual requirements.

Thus, when determining the sphere power for the flat-top segment, identifying the number that represents the close-up portion of the prescription is essential. The value of -1.25 aligns well with the expectations for a segment designed for reading, making it the correct choice for this question.