The Tonopen is a precision electronic intstrument that allows us to measure the pressure of an animal's eye. Many breeds of dogs are prone to glaucoma (an increase in intraocular pressure) which causes pain and can lead to blindness if not caught early. Often the only outward sign is redness of the eyes - which is also true for many other eye diseases. This gives us a quick, easy way to measure the eye pressure so treatment can be intituted early. It is also invaluble in monitoring the treatment for glaucoma.