The Bausch and Lomb keratometer is a one position keratometer that gives readings in dioptric form. It differs from
the Javal-Schiotz in that object size is fixed, image size is the manipulable variable. The reflected rays are passed
through a Scheiner disc with 4 apertures — two of which are used for the focusing of the mires at the fixed telescope
focal distance, the other two for dual prism doubling. The instrument is based on the Helmholtz design which has
two maneuverable prisms aligned vertically and horizontally. This creates two adjustable images in addition to the
original image, one above and one to the left. By adjusting the distance between the eyepiece and the prism, the
effective power of these prisms can be altered. As the distance is decreased, the effective prismatic power decreases.
This decreases the image size along the respective prism alignment, moving the duplicate image closer to the
original. An increase in the eyepiece to prism distance leads to an increase in prismatic shift. As there are two

prisms, each aligned perpendicular to the other, the major and minor axis powers can be measured independently
without adjusting the orientation of the instalment.

In converting the measurements obtained from the corneal surface into a dioptric value, the B&L keratometer uses
the general lens formula (n'-n)/R and assumes an n' of 1.3375 (compared to the actual corneal refractive index of
n'= 1.376). This is a fictional value, which includes an allowance for the small, yet significant, negative power of the
posterior corneal surface. This allows for a readout in both refractive power (dioptres) and radius of curvature
(millimeters).