ZEISS OPHTHALMOMETER

Making the measurement:

In the center of the hollow cross 
there is a dotted line.  Each of 
the solid marks of the line as 
well as each space between the 
dots are equivalent to 1.00 D.   
This can be used to quickly 
estimate the amount of 
astigmatism by noting 
positioned of the solid cross 
on the hollow one after rotating 
the instrument without 
realigning the crosses.