Many persons are not aware of the extent of the field of view which the eye embraces. Vertically it takes in about 100°, whilst horizontally it will take in some 145°, more or less. The field is smaller on the nasal than on the temporal side. When both eyes are used, the combined field of view is larger horizontally, being about 180°. The field of view which is common to both eyes is roughly a circle of about 90°. There is, however, a marked difference in the distinctness with which objects are perceived in the different parts of field of view. On the fovea centralis two dots placed so as to subtend an angle of 60″ will be perceived as double. That is to say, if a piece of paper, on which are two dots 1/30 of an inch apart, be placed 10 feet away from the observer, these dots will be seen as separated, whilst dots (in this case they should be black and of good dimensions) placed half-an-inch apart would still appear as one if viewed at the same distance near the periphery of the retina. In the yellow spot the distance apart of the cones is such that they subtend about the same angle as the dots when they are seen separate, viz., about 60″; that is, they are about 16/100000 of an inch apart, and hence may have something to say to the limit of separation. The field for the perception of colour is different to that for light. The diagrams (Fig. 3) will show the fields in a satisfactory manner. The concentric circles are supposed to be circles lying on the retina corresponding to parallels of latitude on a globe, and are not, therefore, equi-distant when seen in projection. To make these circles it must be imagined that we have a bowl, in the middle of which is a thin rod standing upright and passing through the centre, and another rod attached to it at the centre of the sphere of exactly the length of the radius. If this last arm be opened to make an angle of 5° with the fixed rod, and be twisted round like the leg of a compass against the bowl, it will make a circle, the projection of which will give the innermost circle of the diagram; if opened to 10° it will give the next circle, and so on for every subsequent 10°. The lines passing through the centre are 30° from one another, the line stretching from 360° to 180° being a line supposed to be vertical. By means of an instrument called the perimeter, the field of vision for each eye can be measured. With its aid any small object can be made to fall on any part of the retina by directing the axis of the eye to a fixed point and moving the object along one of the diameters. Suppose we wish to ascertain the field for a white object, a small white disc is moved, say, along the horizontal line, and the angles at which the retina just no longer sees it are noted. This gives two points in the field, and they are plotted on the chart--in Fig. 3 one touches the outside circle, and the other is at an angle of about 65°. The field of vision is next tested