In order to check the front vertex power of a contact lens, the lens should be placed in which of the following positions relative to the aperture of the lensometer?

To determine the front vertex power of a contact lens using a lensometer, the lens must be positioned with the convex side facing the aperture. This correct positioning allows the lensometer to accurately measure how much light is refracted by the lens, as the front vertex power is defined as the power of a lens at its front surface.

When the convex side of the contact lens is directed toward the lensometer's aperture, it simulates the natural position of the lens on the eye, providing a true measurement for its optical power. This setup enables the lensometer to effectively gauge the degree of curvature and refractive index of the lens material, crucial for ensuring optimal vision correction for the wearer.

Other arrangements, such as placing the concave side toward the aperture or adjusting the distance without facing the appropriate side to the instrument, would not yield accurate or meaningful measurements of the lens's optical power. Thus, aligning the convex side with the lensometer aperture is crucial for obtaining the intended measurement of front vertex power.