Answer to b
a. it can only be performed with the patient lying down
b. it is affected by the rigidity of the globe, soften eyeball 
such as in high myopia may be a falsely low reading.
c. corneal abrasion is common.
The SchiØtz tonometer is an indentation tonometer. It is placed
directly onto the cornea after a corneal anaesthetic has taken 
effect. A weight is applied to a plunger on a lever. Additional weights are applied to cause an indentation in the cornea.

 

Answer to c
The non-contact pneumotonometers measures the change in 
the reflected light from the corneal surface that occurs when a puff of air indents the corneal surface.
It measures the time it takes from the initial generation of 
the puff of air to where the cornea is exactly flattened
(in milliseconds) to the point where the timing device stops.
It takes less time for the puff of air to flatten a soft eye than
it does a hard eye. The change in the light reflex is measured 
by a photoelectric diode and translated into a measurement of intraocular pressure by the instrument.