Bausch and Lomb one-position keratometer

Historical Significance: The keratometer measures the curvature of the anterior surface of the cornea. The concept was first devised by Helmholtz in 1856 in which doubling of the reflected images of the corneal anterior surface was doubled using glass plates. Landolt used prisms for this purpose in 1897. Javal and Schiotz used a Wollaston prism (1881) but in this form of the instrument the size of the object reflected by the cornea is varied by sliding two lit targets along an arc. (see cat No 1315 and 1316) and measurements are made in two orthogonal meridians by rotating the sliding arc. The Bausch and Lomb keratometer was devised in 1928. It is a one position keratometer and differs from the Javal-Schiotz in that object size is fixed and 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 manouverable 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 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 instrument.