Corneal thickness is measured by all Except

The Javal-Schiotz model keratometer consists of two illuminated 'mires' (A and B) fixed on a rotatable circular arc (C) and a viewing telescope T. The double images (aa1 and bb1) of the mires (A and B) are formed on the cornea. Keratometry readings are obtained by coinciding the images a1 and b. The readings are noted first in the horizontal meridian and then the arc is rotated by 90o and the readings are noted in the veical meridian. Reference : A K KHURANA Comprehensive Ophthalmology; edition 4, page-554