Corneal endothelial cell count is done by

The specular microscope is a special instrument that allows the doctor and photographer to see and record the corneal endothelial cells. Some instruments touch the front or the cornea. Others are non-contact. These instruments take advantage of a bright specular or mirror like image that is created when the angle of the light for photography is at the correct angle. Since the cornea is clear, only a small amount returns to do the photography. It&;s similar to looking at the inside surface of a drinking glass by changing the angle of light. Image : Specular microscopy Reference :- Parsons diseases of eye 23rd edition