it will stand in any position. The chart is placed upon the hinder surface of the hand-wheel, and rotates with it, so that, in whatever position the quadrant stands, the corresponding meridian of the chart comes opposite to a scale which is fixed behind the hand-wheel. The operator holds a black wand which carries the test object in one hand, while, with the other, he rotates the wheel and pricks the chart. He can thus prick off his observations with rapidity, and can keep the chart constantly under inspection, so that any portion of the field can be brought under examination at pleasure, and can be re-examined at any future time.

In mapping out the field of vision with Priestley Smith's perimeter, a start is generally made above. It is not a matter of any moment, however, at which meridian the chart is commenced. To get the field on the nasal side to the absolute limit, the eye may be turned to fix a point 30° to the temporal side of the fixation spot. "In glaucoma simplex this suggestion has value. Sometimes the light must be greatly reduced to discover either defects within, or encroachments upon, the periphery of the field. Another device sometimes helpful, is to make the patient face a window, and the glare of the light will sometimes bring out a limitation which would not occur in a normal eye" (Noyes).

M'Hardy's perimeter is an excellently devised and well-constructed instrument. Its chief advantage is that it registers the field automatically. This is effected by a most ingenious mechanism, which causes

the test object on the quadrant, and the pointer which pricks out the chart, to move simultaneously.

Schweigger's perimeter (Fig. 60) is a smaller instrument. It can be carried in the hand, and has advantages, therefore, when it is necessary to make observations at the bed

side.

Method of using the perimeter. Having selected a perimeter, proceed to map out the visual field in the following manner :-Carry the test object from the free end of the quadrant towards the centre, and mark upon the chart the point at which it first becomes visible to the patient. Repeat this process along successive meridians, and when all have been completed, remove the chart, and connect with pen and

FIG. 60.-Schweigger's hand
perimeter.

ink the different marks

which have been made upon it. A map of the field of vision is thus displayed, as shown in Fig. 61.

The size of the test object generally used is 10 mm. square, but it is sometimes desirable to employ smaller objects, especially when taking the colour field.

In some cases it is advisable to sweep the field, or parts of it, in circles rather than in meridians; for instance, in

hemianopic or sector-like defects in which the boundary line of the field runs in a meridional direction. The test object may, for such cases, be fixed in a clip on the quadrant, and carried round the field in successive circles.

The test objects used in ordinary perimetric examination subtend an angle of 2° to 4°, and must therefore

FIG. 61. The area within the dark line represents the normal

field of vision.

cover many thousands of retinal elements. The test is thus a rough one compared with those in use for estimating direct vision. Bjerrum has adopted smaller test objects, placed at a greater distance from the patient. Berry, who has had experience with this method, describes it as "capable, in many cases, of affording data of considerable diagnostic importance. A large black screen, 2 metres broad, which can be

let down from the ceiling to the floor, should be used -the screen is most conveniently placed on the wall opposite the space between the windows, so as to get good light all round. It has to be pretty large, as the projection of the blind spot at a distance of a couple of metres, instead of measuring about I in., as on an ordinary perimeter, where the distance of projection is I ft., or 30 cms., measures 7 in. in diameter, and everything else is, of course, in the same proportion. A screen, 2 metres broad, will admit of testing up to about 27° from the point of fixation, if that be in the middle of the screen; while, by removing the point of fixation to the edge of the screen, a larger field can be measured. When the test object is small, the central point of the screen can be used, and this, of course, is the most convenient arrangement. The test objects used by Bjerrum are small circular discs of ivory, fixed on the end of a dull long black metal rod. They are of different sizes, from 10 mm. to I mm. in diameter. The examination is begun with a disc of 10 mm, diameter at the ordinary distance (30 cms.), and afterwards continued in suitable cases with one 3 mm. in diameter at the distance of 2 metres. In the first case, the visual angle is, in the second 20‰, or approximately 2° and 5′ respectively. In the case of the 5′ visual angle, the boundaries of the normal field instead of being as extensive as they are found to be by the ordinary method of examination, average 35° outwards, 30° inwards, 28° downwards, and 35° upwards. Too much weight must not be attached to

what appear to be slight concentric limitations got from an examination by the small visual angle method. It is the irregular limitations, the more or less sectorshaped defects and blind areas (scotomata), which are of the greatest importance. Concentric limitations.

FIG. 62.-Field of vision from a case of glaucoma in which V=38.

Shaded area marks extent of field by ordinary perimeter test; white area field for small object by Bjerrum's method.--BERRY.

are met with by this test as individual peculiarities. Under normal conditions, however, there are never found to be marked indentations, or scotomata, in the diminished field. Variations in illumination, it must be remembered, too, have a somewhat greater influence on the results got by testing with the small images, than in the case of the ordinary perimetric method."