To understand this figure we must enter into the calculation of the angles. We have an eye-distance of 60 mm., a distance of the edges from the cornea 2000 mm., from the nodal points 2007.4 mm., the distance of each edge from the median line 15 mm., the distance of the two edges from each other thus 30 mm. as long as they are in the same plane. We have to determine the angle under which each eye sees the distance of the two edges. A simple trigonometric calculation gives the following figures: If both eyes are in normal position, at 0°, and both edges are in the same plane, 2000 mm. from the corneæ, the angle for each eye is 51' 22". If the left edge is now moved to +5, the left eye sees the distance of the edges at an angle of 51' 25", the right eye under 51' 10", the difference is thus 15"; if the left edge is at +10 mm., the left eye's angle is 51' 29", the right eye's angle 50' 59", the difference 30". If the left edge is moved to -5 mm., the left eye's angle is 51' 18", the right eye's angle 51' 33", the difference 15"; if the left edge is moved to -10 mm., the left eye's angle is 51' 14", the right eye's angle 51' 45", the difference 31". Now we saw that with normal eye-position when the left edge was moved the threshold was +5.93 and -6.97; a difference of 15" to 20" between the visual angles of the two eyes was thus amply sufficient to give a distinct experience of different distance. When the left eye's angle was about 15" smaller than the angle of the right eye, the difference of the retinal images gave a sure impression of the greater nearness of the left edge. If we now bring the eyes into the position of 30°, the angles are of course different when both edges are in the same plane vertical to the direction of regard. If the two edges are in the same plane, the left eye's angle is 50' 59" and the right eye's angle 51' 45", the difference thus 46". If we move the left edge to +5, the left angle becomes 51' 1", the right angle 51' 34", the difference 33". If we move the left to +10, the left angle becomes 51' 4", the right 51' 24"; the difference is thus still 20", and we must move the left edge to +17 mm. to get an equal angle for the left and the right eye. If we move the left to -5, the difference becomes of course larger, the left eye sees under 50' 56", the right eye 51' 55", the difference 59"; and at -10, the left eye has the angle 50' 53", the right eye 52' 6", difference 1' 13". It is hardly necessary to state here the angles for the changes of the right edge or for an eye-position of 15°, inasmuch as the maximum differences bring out our case most clearly. With an eye-position of 15°, the edges at the same plane give angles of 51' 10" and 51'34", that is, a difference of 24"; if the left edge is moved to -5 mm. the difference becomes 38"; if it is moved to -10 mm. the difference is 54"; if the left edge is moved to +5 the difference decreases to 10" and at +10 mm. to 6".