These few illustrations are merely suggestive as to the importance of perimetry in the study of intraocular and cerebral conditions, further reference to the pathological involvements of the field of vision will be found under the various diseases.
Examination of the Field of Vision.-This must be made for each eye separately; the eye to be examined is directed at a fixed point, as it must remain steadily in the same position, while the other eye is closed. There are three methods of determining the field of vision; the simplest, and, at the same time the poorest, is that by using the hand as a test-object. The physician stands in front of the patient, who directs his left eye to the right eye of the physician, the other eye of each being closed. The physician then moves his hand in a plane midway between the patient and himself from the periphery inward over the limits of the field of view. The patient is to tell as soon as he sees the hand, and if his field is normal he should see the hand at the same time as does the physician. This method is only adopted to determine large defects and in those where the central vision is too poor to see smaller test-objects. The field in patients with cataract is usually tested in this way, using a candle-flame in place of the hand.
The blackboard is the second method of determining the field. In this the patient`s head is rested on a support 30 cm. from the board. A chalk mark is made directly opposite the eye to be examined, on which he is to fix his gaze. The chalk is now gradually approached from the edge to the center, and the patient tells at the moment he first sees it. By marking this spot where he first sees the chalk in all directions of the field, and then connecting the points thus determined, we have the field of vision. By using colored chalks we can determine the field for the various colors. This method is also inexact.
The only exact and scientific method of determining the field of vision is that where the projection is upon a hollow sphere. This is now determined by means of an instrument known as the perimeter. The patient`s head is supported on a chin rest, which is so placed in front of a semi-circle that the eye to be examined is situated in the centre of the curvature of the latter. The eye is then fixed upon the middle point of the semi- circular arc, while a test-object, a small white or colored square, is carried along the arm of the semi-circle. The semi- circular arc is marked with a scale of degrees which can be read off, or in the best perimeters is self-registering on a chart attached.
Professor of Ophthalmology in the College of the New York Ophthalmic Hospital; Surgeon to the New York Ophthalmic Hospital. Visiting Oculist to the Laura Franklin Free Hospital for Children; Ex-President American Homoeopathic Ophthalmological, Otological and Laryngological Society. First Vice-President American Institute of Homoeopathy : President Homoeopathic Medical Society of the State of New York ; Editor Homoeopathic Eye. Ear and Throat Journal : Associate Editor. Department of Ophthalmology, North American Journal of Homoeopathy, etc.