The 900 is being produced for those who wish the tonometer to remain permanently on the slit lamp. It is mounted on a pivot on the microscope and for examination is swung forward in front of the microscope. A notch position ensures exact centring of the prism with the left objective. When not in use the instrument is secured in a notch position to the right of the microscope. Observation of the flattened area of the cornea is made through the left eye-piece only.

The illumination device is moved from the left until it just touches the tonometer support and in position the area of contact is illuminated through the prism and is free of reflection. Also there is more space to hold open the patients eye. The priveo support for the 900 tonometer also serves to hold the photographic attachment. When any of these are used the tonometer must first be lifted off its pivot.