_d_ being the distance between the eyes, D the distance of the sitter, and A the angle which the distance between the eyes, = 2.5, subtends at the distance of the sitter. These angles for different distances are given in the following table:— D = Distance A = Angle formed of Camera by the two from the directions Sitter. of the Camera. 5 inches, 28° 6′ 6, 23 32 7, 20 14 8, 17 46 9, 15 48 10, 14 15 11, 13 0 12, 1 foot, 11 54 13, 11 0 14, 10 17 15, 9 32 16, 8 56 17, 8 24 18, 7 56 19, 7 31 20, 7 10 24, 2 feet, 5 58 30, 4 46 36 inches, 3 feet, 3 59 42, 3 25 48, 4 feet, 2 59 54, 2 39 60, 5 feet, 2 23 72, 6 feet, 1 59 84, 7 feet, 1 42 96, 8 feet, 1 30 108, 9 feet, 1 20 120, 10 feet, 1 12 The numbers given in the greater part of the preceding table can be of use only when we wish to take binocular pictures of small objects placed at short distances from cameras of a diminutive size. In photographic portraiture they are of no use. The correct angle for a distance of _six_ feet must not exceed _two_ degrees,—for a distance of _eight_ feet, _one and a half_ degrees, and for a distance of _ten_ feet, _one and a fifth_ degree. Mr. Wheatstone has given quite a different rule. He makes the angle to depend, not on the distance of the sitter from the camera, but _on the distance of the binocular picture in the stereoscope from the eyes of the observer_! According to the rule which I have demonstrated, the angle of convergency for a distance of _six feet_ must be 1° 59′, whereas in a stereoscope of any kind, with the pictures _six_ inches from the eyes, Mr. Wheatstone makes it 23° 32′! As such a difference is a scandal to science, we must endeavour to place the subject in its true light, and it will be interesting to observe how the problem has been dealt with by the professional photographer. The following is Mr. Wheatstone’s explanation of his own rule, or rather his mode of stating it:—