Text-book of ophthalmology . s directly fromthe amount of overlapping of a\ and this purpose plate « does not[Fig. ) constitute a rectangle but a stair figure with six steps so measured off that an overlapping of one step corresponds increase of refractive power of one dioptry. Thus in the case represented in ami 396 the vertical meridian would be 3 D more refractive than the horizontal.[In using the ophthalmometer the arm B is rotated until the meridian is found inwhich the [mages of a and 0 are furthest apart. This is the meridian of least cornealcur
Text-book of ophthalmology . s directly fromthe amount of overlapping of a\ and this purpose plate « does not[Fig. ) constitute a rectangle but a stair figure with six steps so measured off that an overlapping of one step corresponds increase of refractive power of one dioptry. Thus in the case represented in ami 396 the vertical meridian would be 3 D more refractive than the horizontal.[In using the ophthalmometer the arm B is rotated until the meridian is found inwhich the [mages of a and 0 are furthest apart. This is the meridian of least cornealcurvature or first principal meridian. The direction of this is read off on a scale attachedt the instrument, and then the arm is rotated until the images are closest together(meridian of greatest cornet 1 curvature or second principal meridian). The differencein refraction between the two meridians is then measured in the way described. The twoprincipal meridians are almost always at right, angles to each other.—D.]. ASTIGMATISM 897
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